61831108 Physics Qualitative List

7/30/2019 61831108 Physics Qualitative List

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Measurements

When using a digital voltmeter and digital ammeter to determine the resistanceof a wire, state one possible random error which could occur in the use of thedigital meters. How could this error be kept to a minimum? Explain why thevoltmeter must have a resistance much greater than that of the wire, which

resistance it is trying to measure, in order to avoid a systematic error in the useof the voltmeter. [5] N95/III/I

Quantization error (rounding-off error), where the reading is rounded up or down.Hence a reading of 2.3 could actually be anything between 2.25 to 2.35.Quantization errors, like all random errors, can be minimized by taking severalreadings of V and I and finding the average, or by plotting a graph of V versus Iand drawing a best fit line to even out under and over estimations.Additionally, using higher voltages and currents, and digital meters with greaterprecision and more decimal places will also reduce the random error.

The voltmeter has to have a much greater resistance than that of the wire so thatit will draw minimal current away from the wire when connected across it.This is to minimize the systematic error arising from the reduction in potentialdifference across the wire when the voltmeter is connected.

Kinematics

A ball is thrown vertically upwards. Explain how air resistance would affect thetime taken for the ball to reach its maximum height, and the maximum height towhich it rises. J81/I/13

The time will be shorter. Air resistance increases the downward resultant forceand hence the deceleration experienced by the ball, thus bringing to rest sooner.(Newtons 2nd Law of Motion) Without air resistance, v=u-gt hence t=(u-v)/g = u/gsince v=0. But with air resistance, t=u/(g+a), where a is the additional non-constant deceleration due to air resistance.

The maximum height will be lower. Air resistance increases the downwardresultant force and hence the deceleration experienced by the ball, thus bringingit to rest within a shorter distance. (Newtons 2nd Law of Motion). Without airresistance, v=u-2gs hence s=(u-v)/2g = u/2g. But with air resistance,

s=u/2(g+a) where a is the additional non-constant deceleration due to airresistance.

When a train leaves a station, it accelerates from rest along a straight track.Explain why, in practice, trains do not have a constant acceleration even thoughtheir engine power remains constant. [3] N90/III/I

In practice, air resistance increase with speed. (F=kv) The resultant force actingon the train, which is equal to the forward engine force subtract the air resistance,


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thus decreases with increasing speed. Since acceleration is proportional to theresultant force according to Newtons 2nd Law of Motion, the accelerationdecreases with increasing speed.


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When a driver sees the traffic light turn red, he steps on the brake so that the carwill come to a stop. The total stopping distance = thinking distance + brakingdistance. Explain why thinking distance is proportional to speed whereas brakingdistance is not. Describe in words the relationship between braking distance andspeed. [7] J89/II/9

During the thinking phase, the brake has not been depressed hence there is nodeceleration. Hence the speed is constant during this phase. Since distancetraveled is equal to the product of the cars speed and the reaction time ie. s=ut,the thinking distance is proportional to speed. The constant of proportionality isthe reaction time t.

During the braking phase, the brake exerts a retarding force on the car hencethere is deceleration. The speed decreases non-linearly with distance hencebraking distance cannot be proportional to speed. Using the equation v=u-2as s=u/2a. The braking distance is thus proportional to the square of the initialspeed.

An archer tries to hit a target pointing his arrow at an angle to the horizontal.Neglecting air resistance, the angle calculated is 32 degrees. Suggest with areason whether the angle would in practice be larger or smaller than thatcalculated for the arrow to hit the target. [3] N2000/II/2

Larger. In practice, there is air resistance hence the arrow will fall short of thetarget. In order to reach the target, it thus has to aim to hit further than thetarget. The range of the arrow is R=usin2/g. The maximum range occurs when2=1 ie. 2=90 or =45. The closer the angle is to 45, the larger the range.Hence if an angle is larger than 45, it should be decreased in order to increase

the range. On the other hand, if an angle is smaller than 45, it should beincreased. In this case 32 is less than the optimal angle of 45 , hence the angleshould be increased.

Describe qualitatively the motion of bodies falling in a uniform gravitational fieldwith air resistance.

A falling object has 2 forces acting on it. Its weight acts vertically downwards.Drag force due to air resistance acts vertically upwards. Drag force increases withvelocity of the object. When the drag force is equal to the weight of the object,the object is said to achieve terminal velocity.

In the study of kinematics, we normally adopt a coordinate system as a frame ofreference. Explain why this is necessary. [1] IJC Prelims 08/II/1

It helps us define the starting point for measuring any quantity.


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Explain why it is technically incorrect to define speed as distance traveled persecond. Include in your answer the correct statement defining speed. [2] N04/III/1

The second is not the only unit of time measurement, nor is it the smallestdivision of time interval. Distance traveled per second is the average speedwithin a time interval of one second. Speed should be defined as the rate of

change of distance traveled.

State the conditions necessary for the kinematics equations to be applicable.[2]N98/III/1

Constant acceleration and linear motion.

How do you reduce air resistance?

By bending the body forward into a more streamlined shape, the initial horizontal

velocity at which the body is projected at is increased, increasing the horizontaldistance moved.

Dynamics

A ship is moving towards the dock is decelerated by its engine. Explainqualitatively how the law of conservation of momentum applies to this example.[4] N88/II/8

Considering the ship and water to be one system, the decelerating force is an

internal force. Since there is no net extern al force acting on the system, the totalmomentum of the system is conserved. The momentum lost by the ship istransferred to the water.

Describe the circumstances under which a body is said to be weightless. J79/I/1

A body is weightless when its acceler ation is equal in magnitude and direction tothe gravitational field strength at its location. Under such circumstances, thereaction force between the body and the floor will be zero.

Case 1: Body in a lift under free fall near the Earths surface. The bodys

acceleration is g, the gravitational field strength is g. Since a=g, the body isweightless.

Case 2: Body in space station orbiting the Earth. The bodys acceleration is equalto the gravitational field strength at its location. Since a=g, the body isweightless.


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Case 3: Body which is stationary or moving at constant speed deep in outerspace. The bodys acceleration is zero, the gravitational field strength at itslocation is also zero. Since a=g=0, the body is weightless.

Some makes of cars have, as a safety feature, regions at the front and rear whichare designed to collapse on impact. Give a brief physical explanation of how this

design may help to protect passengers from serious injury in the event of acollision. N85/II/2

In the event of a collision, the initial momentum and final momentum of the car ispredetermined by the speed at which it was traveling. However, by collapsingupon impact, the safety regions increase the time for this momentum to change.This reduces the rate of change of m omentum of the car and its passengers.Since the resultant force acting on the passengers is proportional to their rate ofchange of momentum (Newtons 2nd Law of Motion), the resultant force acting onthem is less and they are protected from serious injury.


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A prototype space gun used to launch satellites accelerates the satellite at2.67x104m/s. Suggest why it would be impossible to use this space gun formanned space flights and why it would be advantageous to site the gun on theequator pointing eastwards. [4] N90/III/1

The human body is not able to withstand such a high acceleration. Human pilots

can withstand an acceleration up to a maximum of 9g without blacking out. Anacceleration of 2.67x104m/s will represent a force 2800 times a persons weight,which will not only cause a black out but also crush the astronaut in the satellite,if it were manned.

The Earth rotates about its own axis from west to east. The space gun can makeuse of this initial eastward momentum of the satellite on Earth to fling the satelliteinto space, thus reducing the amount of fuel or explosives needed to launch it intoorbit. As locations at the equator have the largest radius of orbit, objects locatedat the equator move with the greatest speed and kinetic energy compared tothose located at other latitudes. Hence the total change in kinetic energy of the

satellite between the Earths surface and in orbit will also be the lowest.

Describe the concept of weight as the effect of a gravitational field on a mass.

An object which placed in a gravitational field would experience a gravitationalforce. The magnitude of the gravitational force experienced is the weight of theobject. Weight = mass x acceleration due to gravity.

In physics we know that every force is an interaction between two objects. Thus,forces always come in pairs and they all obey Newtons 3 rd law. Also during theinteraction, the principle of conservation of momentum is observed. In the

following examples, describe briefly how the law and the principle are observed.a) A tennis ball is released from rest, falls vertically to the floor and bounces backagain. b) An alpha-particle travels from a great distance directly towards a goldnucleus, which can be assumed to be stationary. The alpha-particle returns alongthe same path without penetrating the nucleus. [8] AJC Prelims 08/III/4

a) By Newtons law of gravitation, the tennis ball exerts a gravitational attractiveforce acting upward on the floor, so the floor exerts an equal and opposite forceacting d ownward on the ball. At the point of impact, the ball exerts anelectromagnetic repulsive force on the floor downward, and the floor exerts andequal and opposite force upward on the ball.

When the ball is released from rest, the total momentum of the ba ll and the earthis zero. When the ball is falling down, it gains momentum. The floor is gainingmomentum upward so that the total momentum of the system is zero andconserved. The upward momentum of the floor is not observed due to the largemass of the earth and its small velocity. At the point of impact where both havezero velocity, the total momentum is zero and conserved. Immediately after the


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collision, the ball moves upwards with maximum momentum in the upwarddirection. The floor will move downward s with momentum so that the totalmomentum of the system is zero and conserved. The downward momentum ofthe floor again is not felt due to the large mass of the earth and its velocity is sosmall that it is not observed. When the ball is moving up, it los es momentumupward. The floor is also losing momentum downward so that the total

momentum of the system is zero and conserved.

b) There is an electric repulsive force acting on the alpha-particle by the goldnucleus and an equal and opposite repulsive for ce acting on the gold nucleus bythe alpha-particle. At the point of closest approach, the alpha-particle exerts thestrongest repulsive electric force on the gold nucleus. The gold nucleus alsoexerts an equal but opposite strong repulsive electric force on the alpha-particle .

At the great distance, the momentum of the alpha-particle is towards the goldnucleus and the gold nucleus has zero momentum. When the alpha-particle slowsdown as it approaches the gold nucleus, its loss of momentum is transferred to

the gold nucleus. The gold nucleus will have velocity, hence its momentum is inthe same direction so that the total momentum of the system is conserved.Immediately after the impact, the alpha-particle moves in the opposite directionwith small speed. Its momentum is in the opposite direction. The gold nucleus willcontinue to move with greater momentum in the other direction so that the totalmomentum of the system is conserved. When the alpha-particle speeds up in theopposite direction, its momentum will increase in the opposite direction . The goldnucleus will speed up in the other direction and its momentum will increase in theother direction so that the total momentum of the system will remain conserved.

When a neutron hits a stationary carbon atom elastically head on, it loses 0.284

of its kinetic energy and retains only 0.716 of its kinetic energy. Discussqualitatively the effect on the kinetic energy lost if the collision is not restricted tohead-on collisions only. Explain why very light particles such as electrons andmassive particles such as uranium nuclei are unsuitable for slowing downneutrons. [10] N89/II/12

If the collision is glancing and not-head on, the kinetic energy lost by the neutronwill be less than 0.284. This is because on ly the component of the neutronsmomentum acting along the radial direction of the stationary carbon atom will beinvolved in the energy transfer. The tangential momentum, on the other hand, willbe totally retained with the neutron. Hence the kinetic energy transferred from

the neutron to the stationary carbon will be less. In order to slow down neutrons,neutrons have to transfer their kinetic energy to the stationary nuclei. The closerthe mass of the two colliding bodies, the greater the amount of kine tic energytransferred. In the case where the masses are equal, as in the case of a neutroncolliding with a proton, 100% of the neutrons kinetic energy will be transferredand the neutron will stop. Electrons and uranium, on the other hand, have massesmuch different from that of the neutron. The mass of the electron is too small


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while that of the uranium nucleus is too large. Therefore, only a small fraction ofthe neutrons kinetic energy is transferred to these nuclei.

State the relationship between the change in linear momentum of an object, theconstant force acting on the object, and the time for which the force acts. [1]N04/III/2

The change in linear momentum p of an object is equal to the product of theconstant force acting on it and the time for which the force acts.

Suggest with a reason why the momentum equations would not necessarily applyif the particles concerned were traveling at speeds close to the speed of light. [1]N01/III/1

At such speeds, relativistic mechanics replaces Newtonian mechanics inpredicting the behaviour of collisions.In an elastic collision, the two bodies become temporarily compressed and remain

in contact for a short time. Explain why both bodies have the same velocity at thetime of maximum compression and why their total kinetic energy at this time isdifferent from the sum of their individual kinetic energies after they havecompletely separated. J87/II/9

At the point of maximum compression, the two bodies are stuck together andmoving as one, hence they must have the same velocity . Some of the kineticenergy is stored as elastic potential energy in the compressed bodies. However,when the bodies separate, they regain their original shape and the elasticpotential energy is converted back into kinetic energy. No kinetic energy is lostsince the collision is elastic. This is unlike a perfectly inelastic collision, whereby

when the two bodies are stuck together and moving as one, kinetic energy is lostpermanently as heat and sound energy, and cannot be recovered.

In order to reduce the number of road traffic accidents, many countries conductresearch into improving road safety. a) One area of research concerns braking.State three factors that affect braking which might be considered by researchers.b) State one other aspect of car safety that could be researched, and suggestbriefly how the research could be conducted. [5] N04/III/2

a) Frictional force between the tire and the road. Skidding due to the tires lockingin abrupt braking situations. Speed of braking action.

b) Shock absorption in the event of collision. Research can be conducted into theefficacy of the crumple zone at the front and back of the car. The material usedshould be stiff enough so that the car does not break apart, yet not so stiff as toresult in an extremely large force of impact. Force sensors placed on dummydrivers can be used to measure the force of injury in simulated collisions.

Forces


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A tennis racket hits a ball. Suggest why in practice it is impossible for a constantforce to be applied to the ball. [2] J97/III/1

As the tennis racket hits the ball, the racket strings undergo extension. The forceexerted by the string increases as the string elongates, according to Hookes Law.

Therefore, it is impossible for a constant force to be applied.Explain why, when ice floating in a jug of water melts, there is no change in thelevel of the water. [2] N02/III/10

Since the ice floats, in accordance with the principle of floatation, the weight ofice is exactly equal to the weight o f the water it displaces . Therefore, the mass ofthe ice is equal to the mass of water it displaces. Thus when the ice melts, it willproduce the same volume of water as the water it displaces. The water formed bythe melted ice will exactly replace the water displaced by the ice. Therefore thereis no change in the level of the water.


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A mass is added to a vertical spring. The string extends according to Hookes Law.However, the energy stored in the spring is only half of the loss in gravitationalpotential energy of the mass. With reference to the law of conservation of energy,explain this observation. N86/II/13

When the spring first extends to its final equilibrium position, the loss in the

masss gravitational potential energy is equal to the sum of t he springs elasticpotential energy and the masss kinetic energy. However, this kinetic energy islater lost due to damping factors such as air resistance. Thus, when the springsettles in its final equilibrium position, it is stationary. The mass has lost all itskinetic energy, and is left with only the springs elastic potential energy.

A thin plastic bag is found to have weight W when empty and pressed flat. Whenthe bag is filled with air at atmospheric pressure and re-weighed, the weight isagain found to be exactly W. Explain why the two results are the same. N83/I/2

When the plastic bag is inflated, it displaces a volume of air equal to its own

volume. According to Archimedes Principle, the bag will experience an upthrustequal in magnitude to the weight of the air that it has displaced . Since thisvolume of air is exactly equal to the volume of air which filled up the bag, theupthrust is exactly the same as the weight of the air injected into the bag. Thetwo additional forces, being equal and opposite, cancels out each other hence thebag weighs exactly W.

Suggest some similarities and differences between frictional forces and dragforces. J79/I/14

Similarities:Both forces are resistive forces which oppose motion.

Both forces result in heat dissipation.Both forces increase with roughness of bodys surface texture.

Differences: Friction is independent of speed whereas drag force increaseswith speed.

Friction increases with body weight whereas drag force is independentof body weight.

Friction is independent of surrounding fluid medium but drag forceincreases with density of surrounding fluid medium.Friction is independent of surface area of the body, but viscous dragforce increases with surface area.

For the car, motion is impossible without friction. Discuss what is meant byfriction and the direction in which it acts on the car. In your answer, suggestanother example where friction is useful. [5] N06/III/1

Friction is a type of force that exists between contact surfaces, and which resistsrelative motion between the two surfaces or the tendency to such motion. In thecase of the car, the frictional force acts in the forward direction of the car. There isfriction at the contact point between the tires and the road. As the wheels rotate


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clockwise, the tires exert a leftward frictional force on the road surface. ByNewtons third law of motion, the road surface will exert an equal but oppositerightward frictional force on each of the rotating tires. This rightward force is theforward force which accelerates the car.

Friction is also useful in braking. When the brakes are stepped on, there is a

tendency for the wheels to stop rotating and rub against the road surface. In thisinstance, the frictional force acts in the opposite dir ection to the cars motion . Thisproduces a deceleration which brings the car to rest.


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A cyclist traveling at a constant speed of 15m/s exerts a power of 240W. Explainwhy a cyclist traveling at a higher constant speed needs to exert a greater power.[4] It is often stated that many forms of transport transforms chemical energy tokinetic energy. Explain why this cyclist is not making this transformation, andexplain what transformations of energy are taking. [5] N99/III/1

Resistive forces opposing the cyclist comprise friction (which is independent ofspeed), and air resistance (which increases with speed according to F=kv, wherek is a constant). Since acceleration is zero, resultant force is also zero. Hencedriving force is equal but opposite to resis tive force . The power exerted is givenby the formula P=Fv where F is the driving force and v his speed. At the sametime, this force is exerted over a greater distance per unit time. Both increases inF and v result in an increase in the power.

Kinetic energy = 0.5mv. When speed is constant, kinetic energy remainsconstant hence there is no transformation of chemical to kinetic energy. Chemicalenergy (in the form of metabolic activity of the cyclist) is used to do work against

resistive forces such as friction and air resistance, and dissipated as heat andsound energy. The heat transfer raises the internal energy of the bicycle, theground and the surrounding air.

State the origin of upthrust.

Upthrust on a body is caused by the pressure difference b etween its upper andlower surfaces.

Describe a situation in which motion is produced without friction being required.[2] N06/III/1

Rockets in space which eject gas to accelerate themselves produce motionwithout friction being required. The principles of their motion are the law ofconservation of momentum and Newtons third law of motion the backwardthrust on the gas produces an equal and opposite thrust on the rocket.

Work, Energy and Power

Distinguish between gravitational potential energy, elastic potential energy andelectric potential energy.

Gravitational potential energy arises from interaction between masses, wherebyforces involved are always attractive in nature. Elastic potential energy arisesfrom interaction between charges, whereby forces involved can be eitherattractive or repulsive. Elastic potential energy arises from deformation of amaterial whereby forces involved serve to stretch or compress the material.


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Discuss the energy changes which take place in the following systems. a) A masssuspended from a spring and oscillating vertically. b) A nucleus undergoingradioactive decay. [8] N07/III/4

a) When the mass is at the top of its oscillation, it has maximum gravitationalpotential energy. As the mass accelerates downward towards its equilibrium

position, it loses GPE, gains EPE and gains KE. Loss in GPE = Gain in KE + PE. Asthe mass passes its equilibrium position, it has lost half its total GPE. Of this GPElost, half is converted into KE and the other half stored as EPE. A fter theequilibrium position, the tension in the spring decelerates the mass. The massslows down and loses KE, continues to lose GPE, but gains EPE. Gain in EPE = Lossin GPE + KE. Finally at the lowermost position, its GPE=0. Its entire GPE lost isconv erted into EPE . In the case where resistive forces are present, the oscillatingsystem will experience damping and progressively lose total energy due to workdone against these resistive forces. This energy will be dissipated as heat, whichthen raises the internal energy of the spring and the surrounding.

b) An unstable nucleus can undergo radioactive decay to emit an alpha particle,beta particle or gamma radiation. In alpha and beta decay, the stationaryradioactive nucleus disintegrates into two parts: a daughter nucleus and either analpha or beta particle. The products have a total rest-mass energy lower than thatof the radioactive nucleus. The lost mass m is converted into energy accordingto Einsteins mass-energy equivalence equation E=mc where c is the speed oflight in free space. As mass is lost, binding energy increases hence the productshave a higher binding energy and are more stable than the radioactive nucleus.Most of this energy is released as the kinetic energy of the products, with theremainder as electromagnetic radiation mainly in the form of gamma radiation.The gamma radiation comes from the de-excitation of the daughter nu cleus

formed in alpha or beta decay. The daughter nucleus, which is in an excited state,de-excites and falls to a lower energy state. The energy difference is emitted asgamma radiation. The energy of the gamma ray photon emitted is equal to thedifference in the quantised energy levels within the nucleus, which areconsiderably larger than those of the orbital electrons.

Circular Motion

Use Newtons laws of motion to explain why a body moving with a uniform speedin a circle must experience a force towards the center of the circle. J78/I/14

When a body moves in a circle, even though the speed remains uniform, itsvelocity is continuously changing due to the changing direction. Newtons first lawof motion states that a body will continue in its state o f rest or uniform motion in astraight line, unless acted upon by an external net force. An external net force isthus required to change the direction of its motion. The direction of change isalways towards the center of the circle. Newtons second law of motion states thatthe net force produces an acceleration in the same direction . Since theacceleration is towards the center of the circle, the force must also be towards the


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center of the circle. They are known as the centripetal acceleration andcentripetal force, respectively.

An aeroplane circling the Earth above its equator is said to orbit the Earth.Discuss the validity of his statement. [2] YJC Prelims 08/III/6

It is not valid. To be in orbit, the acceleration of the aeroplane must be equa l tothe gravitational field strength. The aeroplane would be falling freely but here, ithas a thrust force due to its engine.

A table fans blades broke and you glued them back. Use Newtons laws toexplain why the blades might break off again at high angular velocities. [4]N89/II/9

Newtons first law of motion states that a body will remain in its state of rest oruniform motion in a straight line, unless acted upon by an external net force.When the blades undergo circular motion, their direction is continuously changing.To change the direction of their velocity, a net external force has to be applied.This is the centripetal force, which is provided by the holding force of the gluebetween the blade and the axle of the fan. Centripetal force increas es withangular velocity. At high angular velocities, the centripetal force required exceedsthe holding force of the glue. The blade thus breaks and flies off tangential to itscircular motion, in a straight line uniform motion according to Newtons first law ofmotion.

Using energy considerations, suggest why it is difficult to drive a fairground rideto whirl people on a platform in circular motion at a constant speed. [4] N02/III/2

As the panel rises and falls, it gains and loses gravitational potential energyrespectively. There is a tendency for the platform to lose kinetic energy and slowdown on the way up, and gain kinetic energy and speed up on its way down. Toavoid this, the motors power output has to be increased on its way up andreduced on its way down . To complicate matters, the rate of gain or loss ingravitational potential energy is not uniform throughout the motion, varying fromzero when the arms are vertical, to maximum when they are horizontal. Tomaintain a constant speed would require very precise control of motor power

output using computerized controllers.

Forces acting on bodies which travel in a circle are responsible for the following.Suggest an explanation for each. a) A string may snap if it is attached to an objectand the object is spun around a vertical pole. b) The rings of Saturn are centeredon the center of the planet. [4] N06/III/3


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a) As the string wraps round the pole, the radius of the circular orbit becomesprogressively smaller. Assuming no loss in kinetic energ y of the object due to airresistance, its speed will remain constant. The tension in the string, whichprovides the centripetal force for the objects circular motion is given by T=mv/r.With a constant mass m and speed v but a progressively smaller orbital radius r,the tension becomes progressively greater. The string may snap if this tension

exceeds the strings breaking strength.

b) The gravitational force acting on the rings provides the centripetal force fortheir circular motion about the planet. Since the gravitational force acts towardsthe center of the planet while the centripetal force acts towards the center of theorbit, the plane of the rings orbit must contain the center of the planet. Anyasteroid which orbits about a circle not centered on the center of the planet willexperience a force which moves it towards the equatorial plane.

Gravitation

Why is it incorrect to call g gravity? [2] J94/II/2

Gravity refers to the gravitational force acting on a body. g is not a force, butthe acceleration due to this gravitational force F. Since F=ma, this accelerationa=F/m.

The Moon orbits about the Earth with a centripetal acceleration of 0.00271m/stowards the Earth. Since the Moon is accelerating towards the earth, why doesthe Moon not fall and hit the Earth? J91/III/1

The Moon has a tangential momentum which is perpendicular to the gravitationalpull that the Earth exerts on it. The gravitational pull of the Earth on the Moon isjust sufficient to provide the centripetal acceleration required to keep the Moon incircular motion. The centripetal acceleration keeps altering the direction of theMoons velocity so that it is always tangential to its circular path, and thecontinual falling simply results in a change in the Moons direction of motion .Since the Earth is round, as the moon accelerates towards it, the Earths surfacecurves away at the same angular rate, thus maintaining a constant distancebetween the Moon and the Earths surface. Therefore, the Moon will never hit theEarths surface.

Explain why the center of the Earth is expected to lie on the plane of the moonsorbit. [2] AJC Prelims 08/III/1

The only force acting on the moon is the gravitational force directed towards thecenter of the Earth. The moon will only move in the plane where this force lies .

Explain why gravitational potential has a negative value? [2] N94/III/1


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Due to the attractive nature of the gravitational force, an external agent has toexert a force in the opposite direction to the displacement of the mass, in ordernot to cause it to accelerate when bringing it from infinity to the point in question.Since the applied force and the displacement are in opposite directions, the workdone by the external agent is negative. Since potential is defined as work doneper unit mass by the external agent, potential is negative.

Explain why gravitational field strength at the poles is greater than that at theequator.

Part of the gravitational field strength at the equator is used to provide centripetalforc e for circular motion.

Explain why a geostationary satellite is often used in telecommunications.

Its radius of orbit is always fixed at 4.23x107m. Therefore its position can bepredicted accurately at all times.

What are the advantages of a low orbit satellite?

Time of transmission is shorter. Less energy is required to bring the satellite toorbit. Signal of transmission is higher.

Why must a geostationary satellite be above the earths equatorial orbit?

The center of earth is approximately the c enter of the satellites orbit . Allgravitational force exerted by the satellite provides centripetal force required for

circular motion. There is no net force acting on the satellite.

Explain why the energy at infinity is normally taken to be zero. [1] IJC Prelims08/III/4

The force between the respective two entities is negligible. Thus, the potentialenergy at infinity is zero and it is used as a reference for other potential energy.

State the condition(s) under which the equations to find the gravitationalpotential energy U=mgh and U=-GMm/r may be used. [2] MJC Prelims 08/II/2

The first equation can be used to find the change in gravitational potential energydue to a change in the distance h when distance h is


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Satellites in low orbits are closer to the Earth hence their signals are receivedmore strongly. These satellites can fly over and serve every part of the worldequally. A geostationary satellite allows continuous transmission between theground station and the satellite, since they are always at the same spot above theequator. Less energy is also required to launch and maintain it.

Communications satellites lose speed due to friction. Discuss this consequence inits motion of not correcting for this loss. [2] YJC Prelims 08/III/6

As the satellite loses speed, it would drop to a lower height and it will increase inspeed.

Thermal Physics

State why the thermodynamic scale of temperature is called an absolute scale.

[2] J90/II/6

This means that the temperature on such a scale is not dependent on thethermometric property of any particular substance and has absolute zero as itsminimum temperature. Its fixed point, absolute zero, is not dependent on anyparticular substance but rather is the temperature at which all substances have aminimum internal energy.

List the basic assumptions of the kinetic theory of gases. J77/I/16

A gas consists of a large number of molecules in constant random motion and

which obey Newtons laws of motion.Inter-molecular forces of attraction are negligible.Collisions are elastic on average.The volume of the molecules is negligible compared to the volume occupied bythe gas.Collisions are of negligible duration compared to the duration between collisions.

What experimental evidence is there that the assumption that the collisions of themolecules with the walls of the container are elastic is reasonable? N84/I/17

Collisions are elastic as the temperature of the insulated cylinder of gas is

constant. The temperature of an insulated cylinder of gas remains constantprovided no heat lost to surrounding. Temperature is proportional to the averagekinetic energy of gas molecules. The fact that temperature remains constantshows that collision is elastic. Otherwise, the temperature will keep falling withtime as molecules lose their kinetic energy.

How may an isothermal compression of a gas be achieved in practice? N78/I/15

Heat is removed from the system at the same rate that work is being done on it.


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Two different gases at the same temperature have molecules with different meansquare speeds. Explain why is this possible. [2] ACJC Prelims 08/III/2

Gases at the same temperature have the sam e average translation kineticenergy. However, different gases have different masses , therefore mean square

speed is different.In the upper atmosphere, a large fraction of the hydrogen molecules travel so fastthat they are able to escape from the Earth. There may also be some loss ofhelium, but for all other heavier molecules escape in negligible. Explain a) fastmolecules are able to escape form the Earths atmosphere but slow molecules arenot; b) molecules of smaller mass are able to escape whereas more massive onesare not; c) some molecules escape even though the speed required for escape isvery much greater than their root-mean-square speed. J89/II/10

a) Fast molecules have sufficient kinetic energy to overcome their potential

energy due to Earths gravitational pull. They will escape if their speed exceedsthe escape speed of the Earth, which is approximately 11km/s.

b) For the same temperature, root-mean-square speed is inversely proportional tothe square-root of a molecules mass. Smaller molecules thus have greater rmsspeeds and thus more likely to exceed the escape speed.

c) Root-mean-square speed is juts a form of average speed. Even at the sametemperature, there are molecules with speeds much greater and much less thanthe rms speed. Those that are much faster may exceed the escape speed, andthus escape.


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If two sufficiently energetic sodium atoms collide, an electron in one of them maybe raised to a higher energy level. a) Explain why such a collision is called aninelastic collision. b) Briefly explain why such inelastic collisions are not consistentwith basic assumptions of the kinetic theory of gases. J90/III/3

a) When the sodium atom is raised to a higher energy state, the energy is gained

from the kinetic energy of the colliding sodium atoms. The colliding atoms thuslose part or all of their kinetic energy. Since kinetic energy of the system is notconserved, the collision is inelastic.

b) Kinetic theory of gases assumes that molecules make elastic collisions so thattheir aver age kinetic energy, and hence temperature, remains constant . Ifcollisions are inelastic, then colliding molecules progressively lose their kineticenergy and gradually slow down. The temperature of the gas thus progressivelydecreases, even though no heat is supplied / removed or work done on / by thesystem.

Explain why the specific latent heat of vaporization of a substance is much largerthan the specific latent heat of fusion. N97/III/5

During boiling, the liquid absorbs heat known as the latent heat of vaporization.This heat breaks the intermolecular bonds between molecules and separatesthem to a great distance apart, as well as pushes back the atmosphere. Duringmelting, the heat absorbed, known as the latent heat of fusion, does not break theint ermolecular bonds completely or separate the molecules to a great distanceapart. The heat merely modifies the bonds from a rigid structure to a form thatallows relative motion amongst molecules. Less energy is required. Furthermore,the change in volume in the substance is negligible during melting hence little

work is done on or by the system.

Explain the following observations a) when pumping up a bicycle tyre the pumpbarrel gets warm; b) when a gas at high pressure in a container is suddenlyreleased, the container cools. [2] CJC Prelims 08/III/2

a) External work done on the gas during compression causes the internal energyto increase. Since the internal energy is proportional to the temperature, thetemperature increases.

b) The gas undergoes sudden expansion and internal energy is used do work

against external pressure. This causes the internal energy of the gas to decrease,hence the temperature decreases and the container cools.

Internal energy is determined by the state of the system. Explain what thismeans. [2] PJC Prelims 08/III/1


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A system is at a definite state when it is at equilibrium under a given set ofconditions. The system has a specific pressure, volume, number of moles andmolecules and thermodynamic temperature values that correspond to that state.Internal energy is determined by the values of the current state and isindependent on how the state is arrived at. If a system undergoes a series ofchange from one state to another, its increase in internal energy is the same,

regardless of which path it has taken.

Suggest how an isothermal expansion can be achieved in practice. [2] HCI Prelims08/III/2

The process must be carried out very slowly so that heat has time to flow in or outto maintain constant temperature. The walls of the container must be a goodconductor.

Bubbles are formed in water when water boils. Explain why the bubbles in boiling

water get larger as they approach the surface. [1] PJC Prelims 08/III/1

The pressure of water decreases with a decrease in depth of water. The bubbleexperiences less pressure as it approaches the surface. Since PV=nRT, thevolume of the air bubble is inversely proportional to the pressure it experience.Therefore the volume increases as it approached the surface.

Explain why the melting of ice takes place without a change in temperature. [3]TPJC Prelims 08/II/1

Heat energy that ice absorbs from the surrounding when it is melting is used to

increase potential energy of water molecules. Heat energy has not been used toincrease the average kinetic energy of the water molecules. Thermodynamictemperature of matter is directly proportional to its average kinetic energy , hencemelting of ice takes place without a change in temperature.

Explain what is meant by an ideal gas. N07/III/6

An ideal gas is a hypothetical gas that obeys the equation of state of an ideal gasperfectly at all temperatures and pressures. A real gas approaches idealbehaviour at very low pressures and high temperatures.

Outline how molecular movement causes the pressure exerted by a gas. [2]N99/III4

Gas molecules are in continuous random motion. When a molecule collides withthe walls of a container, it exerts a force which is proportional to its mass, speedand frequency of collision. The force by all molecules acting per unit surface areais the pressure exerted by the gas.


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Outline an electrical method to determine the specific latent heat of vaporizationof water. [4] N98/III/5

An electric kettle, with voltage rating V and current rating I, contains some waterand is placed on a balance. The kettle is switched on and when the water is

boiling, the reduction in the balance m within a time interval t is recorded. Thespecific latent heat of vaporization lv can be calculated from the followingequation: lv=IVT/m. Errors resulting from re-condensation of steam at the mouthof the kettle, as well as heat loss to the surrounding, should be minimized withappropriate measures.

Outline the principles involved in measuring the specific heat capacity of a liquid

using an electrical method. [4] N01/III/5

An insulated container contains the substance which specific heat capacity is tobe measured. An electric heater is placed in the liquid. A thermometer is used tomeasure the temperature of the liquid and a stirrer ensures the temperature isevenly distributed throughout the liquid. The heater is switched on for apredetermined period of time t. The rise in temperature of the liquid within thistime interval is recorded. The voltage V and current I is recorded from thevoltmeter and ammeter respectively. The specific heat capacity of the liquid canbe found from the equation c=IVT/(m ).

State two reasons why the temperature of a body is not a measure of the quantityof thermal energy in the body. [2] N06/II/4

Temperature is a measure of the average kinetic energy of the moleculeswhereas the thermal energy in a body is its total energy. Temperature onlymeasures the kinetic energy of the molecules whereas thermal energy includesboth their kinetic and potential energy .

Superposition

An experiment is conducted to obtain the displacement-time graph anddisplacement-distance graph of a sound wave. Suggest an experimental methodfor obtaining the displacement-time graph. Discuss whether the same methodcould be used for the displacement-distance graph. [6] N96/III/2

The displacement-time graph can be obtained by placing a microphone in thepath of the sound wave, and connecting the output from the microphone to anoscilloscope. The microphone acts as a transducer to convert sound energy intoelectrical energy, such that the amplitude of the microphones voltage output is


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proportional to the amplitude of the sound wave. The x-axis of the oscilloscope isthe time base. The oscilloscope displays voltage-time graph of the microphoneoutput, and thus the displacement-time graph of the sound wave.

The same method cannot be used for the displacement-time graph because theoscilloscopes horizontal axis is time and not distance. Displacement-distance

graph shows the displacement of all the particles along the wave at one instant intime. To produce the displacement-distance graph, multiple microphones must beplaced along the sound wave, and all the microphone outputs connected to adata-logger which records all of their voltage outputs simultaneously . Their valuesare then plotted against distance to obtain the displacement-time graph.


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It is possible to use two separate oscillators feeding two loudspeakers todemonstrate interference of sound. It is not possible to use two filament lamps,however similar, to produce interference of light. Explain the difference. Howcould you modify the experiment in order to show interference of light? [4]J91/III/2

Provided the two sound oscillators are identical, they will maintain a constantphase difference. This ensures that both waves are coherent a condition forinterference to occur. Light from a filament lamp is emitted in random pulses,with different phases emitting from different parts of the filament. Even if twofilament lamps emit the same frequency of light, their wave trains are not able tomaintain a constant phase difference between each other. Since they are notcoherent, the two sources of waves are not able to produce interference. Use asingle light source and place a third slit behind the double slits . The third slitensures that the light waves arriving at the double slits are from a single pointsource and thus coherent. Alternatively, laser light can be used since laser light iscoherent.

Describe an experiment by which the frequency of a stationary wave can beaccurately determined. [5] N90/III/3

Place a light gate transmitter-received pair across an antinode of the vibratingstring. Connect the output of the light gates receiver to a CRO. When the string isnot blocking the light gate, the light gate receivers output is high hence the CROdisplays a high voltage. When the string passes through the equilibrium positionand comes between the light gates transmitter and receiver, the light is blocked.As a result, there is a brief drop in the light gate receivers output, resulting in adip in the CROs voltage display. The time-interval between two successive

voltage drops is half a period. From the period T, the frequency f=1/T.

Explain what is meant by interference with reference to a stationary wave on astretched spring. [3] N95/III/3

When the string is plucked in its center, progressive waves emerge in bothdirections from the center of the string. When the waves reach the ends of thestring, they are reflected by a phase angle of 180 due to the ends being fixed.These reflected waves, traveling in opposite directions, superpose with each otherto produce a stationary wave. At locations where the reflected waves are in phasewith each other, they undergo constructive interference to produce antinodes

with twice the amplitude of vibrations of each wave. At locations where they are180 out of phase with each other, they undergo destructive interference toproduce nodes with zero amplitude of vibration. E nergy is redistributed from theminima to the maxima.

If Youngs double-slit experiment were conducted underwater, how would theobserved interference pattern be affected?


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The speed of electromagnetic radiation underwater will be much slower than thatin a vacuum. The wavelength of the wave will decrease and fringe spacing ofobserved interference pattern will decrease.


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The speed v of a progressive wave is given by the expression v=f. A stationarywave does not have a speed. By reference to the formation of a stationary wave,explain the significance of the product f for a stationary wave. [3] N04/II/4

A stationary wave is formed by the superpo sition of two progressive waves of thesame type with equal speed, frequency, wavelength and amplitude traveling in

opposite directions. The product f in a stationary wave refers to the speed of thetwo progressive waves that superpose to form the stationary wave. Due to thetwo waves having the same speed f but different directions, their velocityvectors cancel out resulting in the stationary wave not traveling in eitherdirection.

Explain why it is usually necessary to adjust either the vibrating length of thestring or the frequency of the vibrator in order to obtain observable stationarywaves on the string. N95/III/3

Both ends of a stretched string are nodes since they are fixed in position and not

able to move. In order for a stationary wave to be set up in a stretched string, thelength of the string must be an integer multiple of 0.5. If the frequency, and thuswavelength, is fixed, then the length of the vibrating string must be adjusted toaccommodate the condition that its length is an integer multiple of 0.5. However,if the vibrating length of the string is fixed, then the wavelength can be adjustedto accommodate the condition, by varying the frequency of vibration, since =v/f.

Suggest why two-source interference of water waves, using a double-slitarrangement, may not be observed when the slits are wide but, when the slits arenarrow, interference is observed. J96/III/3

When the slits are wide, the water waves do not diffract much. Their wavefrontspass straight through the slits and their paths do not cross. Since their wavefrontsdo not meet, the two waves are not able to interfere with each other. When theslits are narrow, the water waves diffract significantly. Their wavefronts spreadout in a semi-circular profile. Where their wavefronts meet, the two waves areable to interfere. At locations where the waves are in phase with each other, theyinterfere constructively to produce intensity maxima. Where the waves are 180out of phase, they interfere destructively to produce intensity minima.

Diffraction gratings can be used to determine the wavelength of electromagneticradiation. Suggest one advantage and one disadvantage of obtaining the

wavelength by using observations of the second-order diffracted light rather thanthe first-order diffracted light. N95/III/3

Second order diffracted beams make larger angles with respect to the centralaxis. The percentage uncertainty in the measured angle will thus be lower.Second order diffracted light has a lower intensity due to the diffraction envelopeconstraint resulting from diffraction of light from each individual slit. This causesthe diffracted beams intensity to be progressively lower the further it is from thecentral axis, making observation of the maxima more difficult.


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Describe briefly how you would confirm that the laser beam is plane-polarised. [2]JJC Prelims 08/III/2

Pass the laser beam through a polariser. As the polariser is rotated in planeperpendicular to the beam, the intensity of the emerging beam should vary from

a maximum to zero or vice versa.Both diffraction and superposition of waves occur when a diffraction grating isused to produce a spectrum. The same principles are involved when a compactdisc is viewed in white light and is seen to produce multicoloured streaks of light.Explain how a diffraction grating produces a spectrum and suggest what thisimplies about the nature of the surface of the CD. [5] N94/III/2

When white light passes through a diffraction grating, the light waves diffract andspread out from each of the many slits. The spreading wave fronts then interferewith one another. At certain angles where the light waves are in phase, they

interfere constructively to produce intensity maxima. These angles where themaxima occur depends on the wavelength of the light according to the equationdsin=n where d is the slit separation and n the order of the maxima. Sincewhite light comprises a whole range of wavelengths, each wavelength of light willhave its maxima at a different angle, thus producing a spectrum.

The fact that the CD is seen to produce a spectrum suggests that the surface ofthe CD has a diffraction grating-like structure. The indentations on the surface ofthe CD must be regularly spaced just like a diffraction grating. Each indentation isa small mirror and reflects white light incident upon it. The reflected light from allthe indentations is similar to the multiple source of light wave spreading out from

the slits of the diffraction grating. They interfere with one another to produce aspectrum as in the case of a diffraction grating.

Light from a laser is produced via stimulated emission, whereas light form afluorescent tube is produced via spontaneous emission. Compare the light fromthese two sources in terms of phase difference and deduce the effect this has onthe experimental setup for a double-slit interference experiment using laser light.[2] YJC Prelims 08/II/3

All photons produced by stimulated emission in a laser have the same phasedifference, whereas those produced by spontaneous emission do not. Thus for a

double-slit experiment, light from a single laser source incident on the double slitswill produce an interference pattern.

Explain why a stationary sound wave set up in a closed tube looks like atransverse wave although sound waves are longitudinal waves. [2] N06/III/4


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This is only a schematic graphical representation of the displacement of the airmolecules in the sound wave. Since sound wave is a longitudinal wave, the airmolecules vibrate parallel to the axis of the pipe. The air molecules actuallyvibrate in a horizontal direction (left and right) but the schematic graphicalrepresentation is such that a rightward displacement is represented as positivewhile a leftward displacement is represented as negative.

The band is practicing in the band room down the corridor with the door leftslightly ajar. Explain why notes of a certain range of frequencies can be heardmore clearly than others at the end of the corridor. [2] HCI Prelims 08/III/7

Low-pitched notes have longer wavelengths and undergo more diffraction throughthe door.

Oscillations

Some sand is placed on a flat horizontal table and plate is made to oscillate withsimple harmonic motion in a vertical direction. With the frequency of oscillationkept constant at 13Hz, the amplitude of oscillation of the plate is graduallyincreased. Explain at which point the sand loses contact. [3] J99/III/4

The sand starts to lose contact with the oscillating table when the maximumdownward acceleration of the table exceeds the acceleration of free fall of thesand. Maximum downward acceleration occurs at the maximum upward

displacement position. When the amplitude of oscillation exceeds 1.47mm, themaximum acceleration exceeds that of free fall. When this happens, at itsmaximum displacement position, the table accelerates downwards with greatermagnitude than the sands acceleration due to free fall. This will cause thesubsequent downward velocity of the table to exceed that of the sand. Thecontact force between the sand and table falls to zero and the sand loses contactwith the table.

A block of wood of mass m floats in still water. When the block is pushed downinto the water, without totally submerging it, and is then released, it bobs up anddown in the water with a frequency f. Surface water waves are then incident on

the block. These cause resonance in the up-and-down motion of the block.Describe and explain what happens to the amplitude of the vertical oscillations ofthe block after the following changes are made. a) Water waves of largeramplitude are incident on the block. b) The distance between the wave crestsincreases. c) The block has absorbed some water. [6] N94/II/2

a) The amplitude of the block increases. The energy of a wave is proportional tothe square of its amplitude. Waves of larger amplitude possess greater energy ,thus transferring a greater amount of energy to the block.


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b) Resonance no longer occurs hence the amplitude of oscillation decreases.Distance between wave crests is the wavelength . Since speed of wave v=f andv is constant, a larger means a lower frequency f . The frequency f of the drivingforce drops below that of the natural frequency of the block.

c) Resonance no longer occurs hence the amplitude of oscillation decreases.When the block absorbed some water, its mass increases. The natural frequencydecreases. The frequency of the driving force is now above the natural frequencyof the system.

State one situation in which resonance is used to advantage. [1] J97/II/2

Radio tuners natural frequency adjusted to resonate with frequency of radio waveof the station it intends to receive.


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A mass-spring system is used to demonstrate the effects of damping on theoscillation of the mass. Suggest how light damping of the oscillations may beachieved and how the degree of damping may be increased. [4] N2000/II/3

Light damping may be achieved by attaching a light piece of cardboard to themass thus increasing its effective area and air resistance experienced by it. The

degree of damping may be increased by immersing the mass in a viscous liquidsuch as glycerine, which provides a large viscous drag force against the motion.This causes large amounts of energy to be dissipated from the body.

Discuss the behaviour of a car when the shock absorber mechanism is workingcorrectly. N78/III/1

A shock absorber mechanism should be critically damped. When a car goes over ahump, there is an immediate compression of the suspension springs. A properlyworking mechanism should then return the spring to its uncompressed position inthe shortest time, w ithout overshooting the equilibrium position.

Resonance can cause considerable engineering problems. Explain one suchproblem and suggest ways in which it can be overcome. [4] N98/III/2

When resonance occurs, the system vibrates with maximum amplitude an denergy. This vibration creates tremendous stress on mechanical structures andmay cause them to breakdown. One such problem is in building and bridgestructures. If the frequency of the gusts due to high speed winds coincides withthe natural frequency of the bridge, the bridge may resonate and vibrate withmaximum amplitude. This problem can be overcome by increasing the damping ofthe system through the use of shock absorbers.

Describe 2 practical examples of forced oscillations and resonance.

Microwave oven Microwaves will cause the water molecules in the food tovibrate. If the frequency of the microwave is the same as the natural frequency ofthe water molecules, resonance occurs and the water molecules will vibrate withmaximum amplitude, hence heating up the food.

Breaking of a glass by a singer sound waves from the singer will cause the glassmolecules in the glass to vibrate. If the frequency of the sound wave is the sameas the natural frequency of the glass molecules, resonance occurs and the glass

molecules will vibrate with maximum amplitude, and hence may break the glass.

If a truck travels at a certain speed over a series of speed bumps, the verticaloscillations can be very large. Explain why this is so. [2] MJC Prelims 08/II/2

Reso nance occurs. The frequency of passing over the bumps matches the naturalfrequency of the truck.


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Damping is very often necessary when unwanted oscillations occur in machinerysuch as a washing machine or a dishwasher. Describe the factors that determine

the frequency at which unwanted oscillations occur and the principles that areused in damping out these oscillations. [5] H2 Specimen Paper 07/III/6

The frequency of unwanted oscillations is the natural frequency of the system.The natural frequency of a system is determined by factors such as the inertia ofthe system (mass), the elastic restoring force acting on it (spring constant) andthe dimensions of the system. For machinery like washing machines anddishwashers, their mass, springiness of their metal frames and dimensions of theircasings determine their natural frequency and frequency of unwanted oscillations.Damping involves the reduction of amplitude of an oscillating system through theapplication of external resistive forces that results in energy being dissipated from

the system. This can be achieved by using dampers. Dampers can be in the formof viscous fluids or solid material with dissipative properties attached to the partsof the machine that are likely to experience the most vibrations.

Waves

The wavelength of sound in air is of the order of one million times greater thanthe wavelength of light in air. Describe how you could check this statementexperimentally. [8] N04/III/3

The wavelength of a wave can be determined using two-source interferenceexperiments. To determine the wavelength of sound, connect two loudspeakers toa single sound oscillator. Face the two speakers towards a wall several metresaway. Move a microphone along the wall to detect variations in sound intensity.There will be intensity maxima corresponding to constructive interference of thetwo sound waves, and minima corresponding to destructive interference. Select amaximum that is nth order from the central axis maximum. Measure the distancefrom each speaker to this maximum and subtract them to obtain the pathdifference. The path difference divded by n will be the wavelength of the sound.

To determine the wavelength of light, use Youngs double-slit experiment. A

monochromatic light source is placed behind the double slits, with a third slitbetween them to ensure coherence. Bright and dark fringes will be observed onthe screen. Since the angle of diffraction is small in this experiment, the Youngsdouble-slit formula y=D/d can be used to calculate the wavelength of the light.The experimental results will show that the wavelength of sound waves is in theorder of metres; while that of light waves in the order of micrometres.


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Distinguish between the displacement of a vibrating object and the amplitude ofvibration. [3] J99/III/4

Displacement is the shortest distance from the equilibrium position to the objectat any instant in time. Amplitude is the maximum displacement of the object fromits equilibrium position, in either direction.

What phenomenon associated with transverse waves is not observed withlongitudinal waves? N90/III/3

Polarisation. Transverse waves vibrate perpendicularly to its direction ofpropagation, and possess many possible planes of vibration. Polarisationeliminates all plan es of vibration except the plane which is parallel to the allowed

axis of transmission of the polarizer. Longitudinal waves vibrate parallel to itsdirection of propagation, and possess no plane of vibration parallel to the allowedaxis of transmission of the polarizer. Therefore, longitudinal waves cannot bepolarized.

Electricity

Discuss briefly whether it is practicable to suggest that power losses as heat canbe reduced indefinitely by a suitable choice of generator pd. N81/1/7

Theoretically, power loss can be reduced infinitely by increasing the generator pdby using step-up transformers at the generator output. However, extremely highvoltages tend to cause insulation breakdown between live and neutral wires /insulation breakdown between windings in the high-voltage secondary coil of thetransformer / electromagnetic interference with the telecommunicationequipment. Hence, there is a limitation to how high the voltage can be steppedup.

Why do the I-V graphs of a filament of a torch bulb and a piece of intrinsicsemiconductor differ in shape? N89/III/5

The resistance of a metallic conductor increases with temperature. As the voltageand current increase, the power dissipated increases causing significant heating.This increases the ionic lattice vibration within the metal, causing increasingcollisions with the drifting free electrons. The drift velocity of the electronsdecreases; the filaments conductivity falls; and resistance rises. Since R=V/I, thisratio increases thus causing the graph to concave downwards.


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The resistance of a semiconductor decreases with temperature. As the voltageand current increase, the power dissipated increases causing significant heating.This energy causes more electrons to be released from their bonds. The chargecarrier density of the semiconductor increases; its conductivity rises; andresistance falls. Since resistance R=V/I, this ratio decreases causing the graph toconcave upwards.

By taking the average value of power dissipated at different temperatures,energy supplied by battery during period which temperature changes can becalculated. However, this calculation is only an estimate. Explain.

Since power does not vary linearly with resistance, this method will not yield anaccurate value of power dissipated when resistance changes.


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Explain why power is required to maintain an electric current through a metallicconductor. [4] J98/III/2

The flow of current is a result of electrons drifting in the opposite direction.Electrons drifting through the conductor constantly collide with the metal ions inthe lattice, passing energy to the ions, causing them to vibrate, and resulting in a

rise in the temperature of the conductor. In order to prevent the electrons fromslowing down after the collision, a continuous amount of energy must be suppliedby the potential difference across the conductor. This potential difference createsan electric field which immediately accelerates the electrons back to theirterminal drift velocity after each collision with the ionic lattice. In this way, thepower supplied by the potential difference balances the power dissipated as heatby the vibrating ionic lattice, in accordance with the law of conservation ofenergy.

In using a simple slide-wire potentiometer circuit, a large protective resistance issometimes connected in series with the galvanometer. Why is this done? Explain

how the presence of this resistance affects a) the position of the balance point, b)the precision with which it may be found. J84/I/7

This resistance lim its the current flowing through the galvanometer, preventing itfrom being damaged. When the potentiometer is still far from its balance point,there could be a large potential difference across the galvanometer. Since thegalvanometers resistance is low, the current flowing through it could beextremely high if not limited by the protective resistance. This protectiveresistance does not affect the position of the balance point. At the balance point,there is no potential difference across the galvanometer. The current will thus bezero whether or not the protective resistance is present. However, this protective

resistance reduces the precision with which the balance point may be found. Thisresistance reduces the current through the galvanometer, thus making it lesssensitive to small changes in potential difference. To overcome this problem, adual-resistor system is used whereby the protective resistance is switched to alower value when the jockey is near the balance point. By then, the potentialdifference across the galvanometer will be quite small.

Explain the use of a thermistor and a light-dependent resistor in potential dividersto provide a potential difference which is dependent on temperature andillumination respectively.

A thermistor is sensitive to temperature changes. When temperature increases,resistance decreases. Hence current from the DC source will increase. Hence, thepotential difference across the resistor increases. A light dependent resistor issensitive to illumination. When illumination increases , resistance decreases.Current from the DC source will increases. Hence, the potential difference acrossthe resistor will increase.


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Use Faradays law of electromagnetic induction to explain why a transformer willoperate for an alternating voltage input but not for a direct voltage input. [2] AJCPrelims 08/III/3

An alternating voltage input causes a changing magnetic flux which links theprimary coil to the secondary coil. As a result of electromagnetic induction, a

changing emf is produced in the secondary coil. A direct voltage input does notprovide a changing magnetic flux and thus, no emf is induced in the secondarycoil.

In a potentiometer setup, the instructions suggested that the galvanometer beset to different resistance levels (high or low) for the initial estimation of balancelength and for the final, more accurate verification. Suggest which resistancelevels should be chosen for each measurement. Explain your answer. [4] AJCPrelims 08/III/6

Initially, high resistan ce is necessary to protect the meter against possible high

current. For final verification, a change from high to low resistance means thatthere will be a current flowing through the galvanometer even at the balancepoint that has been determined initially. Hence, a more accurate balance pointcan be determined with fine adjustment till the galvanometer reflects nulldeflection again.

If the equation V=IR is used to define resistance, why is it not possible then to usethe same equation to define potential difference? [1] CJC Prelims 08/II/4

Resistance of a resistor is defined as the ratio of potential difference across theresistor to the current through it. Therefore, potential difference cannot be

defined using resistance. This is because physical quan tities must be defined insequence.

Explain why a railway employee who touches the track through which there is acurrent of 180A does not get an electric shock. [2] N98/III4

This is because there is no potential difference between the railway employee andthe track. Electric current flows from a region of higher to lower electric potential.Both the railway employee and the track are in contact with the earth, and thusare grounded at the same earth potential. Since the employee and the track areat the same potential, current will not flow either way.

Explain why a high voltage supply is essential for a railway system. [2] N98/III/4

High voltages result in lower transmitted current, since I=P/V where transmittedpower is a constant. A lower current re duces the power loss as Ploss=IR where Ris the resistance of the transmission cable. A lower current either increases the


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efficiency of transmission or, if efficiency is to be maintained, allows a thinnerwire with higher resistance to be used, thus saving on cabling costs .

Explain why a different current is needed when the train is climbing a hill fromthat required when traveling at the same speed on the flat. [2] N98/III/4

When climbing a hill, additional work needs to be done against gravity which isconverted into potential energy of the train. Engine power P=IV=Fv where F isforce exerted and v the speed of the train. Since speed v is constant but a greaterforce F has to be exerted to overcome gravity, a greater current I has to be drawnto provid e the additional power required. This is in accordance with the law ofconservation of energy.

Describe electrical conduction in a metal. [1] N99/III/5

When a potential difference is applied across the ends of a metal, an electric fieldis set up. This field exerts an electric force on the free electrons, causing them to

drift towards the higher potential end.Discuss two factors which affect the magnitude of the current and hence result inthe possible danger of electric shock. One obvious safety precaution is to keeplive wires well insulated. What other safety precautions do you suggest? [4]N2000/III/4

One factor is the resistivity of the skin, which is affected by its moisture. Wet skinhas lower resistivity and hence resistance, thus increasing the danger fromelectric shock. Another factor is the thickness of the skin. If the skin is thinner, theresistance will also be lower, increasing the danger from electric shock.

The other safety precautions are 1) wear thick rubber boots to reduce currentflowing through the body into the earth in the event of an electric shock; 2)connect an Earth Leakage Circuit Breaker in the electric circuit to cut off thepower supply the moment current is diverted into the earth wire.

Electric Fields

Explain why the electric potential on the surface of an isolated charged conductoris the same everywhere. [4] J88/III/5

The surface of a conductor has zero resistance, hence charges on it can flowfreely. Any potential difference which develops between two points on the surfacewill create an electric field between the two points. The field will produce a forcewhich causes the charges to move and redistribute themselves until there is nomore potential difference between the two points. Hence the electric potential onthe surface must be the same everywhere.

Explain to what extent an isolate, positively charged conducting sphere may beconsidered to act as a point charge. [3] J89/III/12


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An isolated, positively charged conducting sphere may be considered to act as apoint charge at distances equal to or greater than the radius of the sphere . On thesurface as well as outside the sphere, the electric field strength E=Q/40dandelectric potential V=Q/40d, where d is the distance measured from the center ofthe sphere. Hence, the sphere behaves like a point charge of magnitude Q located

at its center. Within the sphere, however, E=0 and V=Q/40R everywhere in thesphere, where R is the radius of the sphere, which is a constant. Hence within thesphere, it does not behave as a point charge. The reason behind this is that theresistance of a conductor is zero and there can be no potential difference betweenany two points in the sphere. Since electric field strength is the negative of thepotential gradient, the electric field must be zero everywhere inside the sphere.

Distinguish qualitative and quantitative aspects of electric field and gravitationalfield.

Both are inverse square law fields. Electric field acts on charges while

gravitational field acts on masses. Field lines in an electric field are alwaysdirected away from a positive point charge and towards a negative point charge.Field lines in a gravitational field are always directed to a point mass. Electricforces can be attractive or repulsive while gravitational forces are alwaysattractive.


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Suggest why, in the alpha-scattering experiment, a) gravitational effects areignore; and b) when calculating electric potential, the direction of approach of thealpha-particle to the nucleus need not be considered. [2] N98/II/2

a) The potential energy and force due to a gravitational field are many orders ofmagnitude sma ller than the potential energy and force due to an electric field,

respectively.

b) Electric potential is a scalar quantity which has only magnitude but notdirection. It is hence independent of the direction of approach.

Explain why lightning is more likely to strike the tip of the lighthouse than anyother places close to it. [1] IJC Prelims 08/III/4

Near the tip of the lighthouse, the equipotential surfaces are very close to oneanother. This means that the potential gradient there is very strong and it is morelikely to reach the threshold potential gradient before any other places for an

electric discharge to occur. There is a high concentration of charges at the tip ofthe lighthouse.

Explain the shape of the path followed by electrons as they pass between thedeflecting plates.

A uniform electric field acts from the upper plate to the lower plate, hence aconstant force exerted on the electron beam causes it to curve up. The curve isparabolic.

State how the magnitude and direction of the electric field strength at any point

between two charges can be determined by their electric potential-distancegraph. [2] RJC CT1 08/16

The magnitude of the electric field strength at any point between the two chargescan be obtained from the gradient of the potential-distance graph. The electricfield strength acts from region of high to low potential.

Magnetism

A copper disc spins freely between the poles of an unconnected electromagnet.

Describe and explain what will happen to the speed of rotation of the disc whendirect current is switched on in the electromagnet. [4] J91/II/5

The copper disc will slow down and then come to a halt. When a direct current isswitched on, a magnetic field is created perpendicular to the plane of the copperdisc. The rotat ion of the disc causes the disc to cut these magnetic field lines.According to Faradays law of electromagnetic induction, a change in themagnetic flux linking the disc will result in an emf induced across its axle and rim.This emf will cause a current to flow in the disc. According to Lenzs Law, this


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current will flow in such a direction as to oppose the change in magnetic fluxlinkage, hence creating an opposing force. This opposing force will slow down thedisc. By the Law of Conservation of Energy, the mechanical energy of the rotatingdisc is converted into electrical energy and then dissipated as heat in the disc.

State the effect on the magnetic flux density along the axis of a solenoid in the

presence of a ferrous core. [1] H1 Specimen Paper 06/II/4

In the presence of a ferrous core, the magnetic flux density will be increased afew thousand times since the permeability of iron is approximately 5000 timesthat of free space. There will be many more magnetic field lines which are moreclosely spaced compared to before.

A step-down transformer is used to charge a rechargeable battery. The secondarycircuit consists of a diode, a resistor, and the battery to be recharged. a) Suggestwhy the diode is necessary in the secondary circuit. b) Suggest why the resistor isnecessary in the circuit. [4] J2000/II/5

a) The diode ensures current is always in one direction so that battery will notdischarge. There will be times when the charging voltage falls below the emf ofthe battery. During such instances, the diode prevents the current from reversing,out of the battery into the transformer which will cause the battery to bedischarged.

b) The resistor limits the current in the circuit to protect the battery and thediode. This is because the resistance of the diode and internal resistance of thebattery are both very small. If there is no resistor to limit the current, the currentcould become very high, causing damage to the battery and diode.

A long bar magnet is suspended from a helical spring so that one pole of themagnet lies within a short cylindrical coil. The magnet is given a small verticaldisplacement and is then released so that one pole of the magnet oscillateswithin the coil. Explain the nature of the magnets oscillations when no resistor isconnected; a large resistor is connected and a low resistor is connected. J94/III/3With no resistor, although an emf is induced across the coil due to Faradays lawof electromagnetic induction, the current cannot flow hence no magnetic field canbe created to oppose the motion of the oscillating magnet. The magnet thusoscillates freely with constant amplitude.

With a large resistor, the induced emf is able to drive a small current through thecoil. According to Lenzs Law, the current will flow in such a direction as to opposethe change in magnetic flux linkage. This opposing field slows down the oscillatingmagnet. Light damping occurs.

With a small resistor, a large current is able to flow. The opposing field is strong,and the magnet experiences heavy damping.


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In the second and third case, the mechanical energy of the oscillating magnet isconverted into electrical energy, then dissipated as heat in the resistor inaccordance with the Law of Conservation of Energy.

Describe and analyse deflections of beams of charged particles by uniformelectric and uniform magnetic fields.

Beams of charged particles entering a perpendicular uniform electric field aredeflected in parabolic paths. Beams of charged particles entering a perpendicularuniform m agnetic field are deflected in circular paths. The direction of thecentripetal force is given by Flemings left hand rule.


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A coil, consisting of many turns of insulated metal wire wrapped around a soft-iron core, is connected in series with a battery, a switch and a lamp. State whathappens to the magnitude of the magnetic flux in the coil as the current increasefrom zero when the switch is closed. Hence explain why an emf is induced in thecoil as the current increases. Hence explain why there is noticeable delay beforethe lamp lights up after the switch is closed. State and explain what will happen

to the length of the delay if the soft-iron core is replaced by one made of wood.[6] J95/II/14

The magnetic flux increases.

According to Faradays law o f electromagnetic induction , an emf will be induced inthe coil when there is a change in the magnetic flux linking the coil.

According to Lenzs law, the induced emf will be in such a direction as to opposethe change. The induced emf is initially equal but opposite to the batterys emf,thus canceling it out, resulting in there being no net emf in the circuit. Gradually,

this induced emf decreases due to the reduction in the rate of change of magneticflux, thus allowing the battery to drive a current through the circuit.

The delay will be shorter. Wood does not encourage magnetic flux as well as soft-iron. Therefore, the change in the magnetic flux linkage in the coil will be lower.This induced emf which opposes the batterys emf will be thus lower. Hence thedelay will be shorter.

An electron is projected in a vacuum along the axis of a current-carrying solenoid.Describe and explain its motion. N81/I/8

The electron will pass straight through undeflected, without any change in speed.The magnetic field in a solenoid is parallel to its axis. When an electron travelsalong the axis of the solenoid, the angle between the direction of the magneticfield and that of the electrons velocity is zero. Therefore, the force acting on theelectron will also be zero. With no resultant force acting on the electron, it will notundergo any acceleration and thus will travel with uniform speed in the samedirection, in accordance with Newtons first law of motion.

Explain the forces between current-carrying conductors and predict the directionof the forces.

Two parallel current-carrying conductors are placed near to each other. Current inthe first conductor generates a magnetic field in its vicinity whose direction isgiven by the right hand grip rule. The second current carrying conductor withinthis field would experience a magnetic force, whose direction is given byFlemings left hand rule. 2 parallel conductors with currents flowing in the samedirection will attract each other. 2 parallel conductors with curr ents flowing inopposite directions with repel each other.


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Briefly explain the working principles of a current balance. [2] NJC Prelims 08/III/6

The magnetic field produced a magnetic force which results in an anti-clockwisemoment about XY to counter th e anti-clockwise moment due to the weight of therider.

Explain simple applications of electromagnetic induction.

Turbine generator the coil of the turbine is positioned in a strong magnetic field.The turbine uses force to turn its coil. Hence, its coil experiences changingmagnetic flux linkage. According to Faradays law, emf is induced.

Electric guitar the vibrating string induces an emf in the coil. The pickup coil isplaced near the vibrating guitar string which is made of a metal that can bem agnetized . The permanent magnet inside the coil magnetizes the portion of the

string nearest the coil. When the guitar string vibrates at some frequency, itsmagnetized segment produces a changing magnetic flux through the pickup coil.The changing flux linkage induces emf in the coil. Subsequently the emf is fed tothe amplifier and speaker system to produce sound.

By considering the Earths magnetic field, suggest with reasons why Aurorae aremostly seen in the polar region rather than in the equatorial region. [2] ACJCPrelims08/II/7

The direction of the entry of the solar wind (or charged particles) and the Earthsmagnetic field are almost perpendicular at the Equator, hence deflected away

from the Earth. However, the direction of solar wind is almost parallel to field linesat the poles. Hence it is not deflected. Thus Aurora sightings at the equator is rarebut not at the poles.

A charged particle enters a region of uniform magnetic field directed into theplane of the paper. The tracks of such particles can be made visible by anapparatus known as a bubble chamber, in which the particle is made to passthrough a liquid close to its boiling point. Thus tiny bubbles are formed along thepath of the partic

61831108 Physics Qualitative List

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