Dynamics and Space

Newton’s laws

Learning Outcomes• Applications of Newton’s laws and balanced forces to explain

constant velocity, making reference to frictional forces.• Calculations involving the relationship between unbalanced

force, mass and acceleration for situations where more than one force is acting.

• Calculations involving the relationship between work done, unbalanced force and distance / displacement.

• Calculations involving the relationship between weight, mass and gravitational field strength during interplanetary rocket flight.

• Newton’s second law and it’s application to space travel including rocket launch and landing.

• Newton’s third law and its application to explain motion resulting from a ‘reaction force’.

• Use of Newton’s laws to explain free-fall and terminal velocity.

Lesson 1• Define the term friction and give

examples of how to increase and decrease frictional forces.

The Force of Friction• A force of friction is a force acting

between two surfaces in contact which opposes the motion of the surfaces (i.e. tries to stop them moving).

Reducing Friction• As friction always stops things

moving it is important to reduce it if we want to move things.

• We are now going to look at ways of reducing friction.

Demonstrations

Reducing Friction (copy out note or make your own mindmap)• There are four main ways in which we can

reduce friction:1. Streamlining – changing the shape to reduce air

resistance e.g. a car with a ‘flatter shape’ 2. Rollers – adding rollers or wheels e.g. a trolley

with wheels moves easier than one without.3. Lubrication – adding a layer of liquid or air

between two surfaces e.g. oiling hinges to stop them ‘squeaking’.

4. Smoothing – making a surface smoother e.g. adding wax to skis

Increasing Friction• Friction can also be useful to us:1. When a car applies its brakes it

increases friction to slow down.2. Parachutes are used to increase

air resistance to slow things down.Highest Skydive EVER

• 2nd Highest Parachute Jump

Homework• Research a machine or animal that

has been specially adapted to become more streamlined.

• Present your findings in one of the following ways:– a short essay in your homework jotter– an information leaflet– A poster

Lesson 2• Define the term friction and give

examples of how to increase and decrease frictional forces.

Balanced Forces• Today we are going to look at the

effect of two opposite but equal forces acting on an object.

• Tug of war

Balanced Forces• Balanced forces are equal forces acting in

opposite directions.• They are equivalent to no force at all acting on an

object.• When an object is stationary, or travelling at a

constant velocity, all forces on it are balanced.

• (HINT: this is often an exam question – they will tell you an object is travelling at constant velocity with a thrust force of say 100N. They will then ask you what the force of friction would be. This would be the same as the thrust, in this case 100N.)

Balanced Forces: Motion• When a car is moving at constant speed,

the forces acting on it are said to be balanced.

• In the car below, force of engine = friction

Other examples include:Plane

• Note there are also balanced vertical forces, lift = weight.

Other examples include:Boat

• Upthrust = weight

Newton’s First Law of Motion

• Newton’s first law states that if an object is stationary or moving at a constant speed then the forces acting on it are balanced.

Newton’s First Law in action

• Newton’s first law shows us that when you move at constant speed it has the same effect as not moving at all.

• You only feel a force if you are speeding up (accelerating) or slowing down (decelerating).

In an airplane• As the plane takes off you are

pushed back as the plane speeds up.• The forces are not balanced.

In an airplane• As the plane lands you are pushed

forward as the plane applies its brakes and slows down.

• The forces are not balanced.

In an airplane• When in mid-air, going at a constant speed and a

constant altitude, without any turbulence you would feel exactly as you would if you were on the ground

• The forces are balanced. You can do anything that you could do if you were on land.

• Harlem Shake Frontier Flight 157

What about now?• Even now as you sit in

the class (in the UK) you are actually travelling at around 1000 km/h (622 mph) as the Earth rotates on its axis.

• You don’t feel this effect as it is a constant speed.

• (Earth travels at around 1600 km/h at the Equator).

Motion of a spaceship• The motion of a spaceship is

consistent with Newton’s First Law.• Space is a vacuum so there are no

frictional forces acting.• The ship continues moving at the

same speed in the same direction until it comes under the gravitational influence of a planet.

Homework - podcast• Using iTunes, search for the free

podcasts called “stuff you should know” (from howstuffworks.com).

• Find the episode from 21 Feb 2013 on ‘What would happen if the Earth stopped spinning’.

• This will be discussed in a future lesson.

Lesson 3(usually two periods)

• Investigate the relationship between unbalanced force, mass and acceleration.

• Carry out calculations involving the relationship between unbalanced force, mass and acceleration for situations where more than one force is acting.

Newton’s Second Law of Motion

• Newton’s first law of motion looked at how balanced forces affected the velocity of an object.

• We are now going to look at how unbalanced forces change the velocity of an object e.g. acceleration.

Newton’s Second Law of Motion

experiment 1• A light gate is used to measure acceleration of a trolley.• Hanging masses are used for the unbalanced forces.

• Note: the mass of the trolley is kept constant. Only the masses on the hanger are changed causing and unbalanced force (due to gravity acting on the masses).

Results

Force (N) Acceleration (m/s2)

123456

Newton’s Second Law of Motion

Conclusion: As the Force on a body increases so does the acceleration.

In other words a ~ F

Newton’s Second Law of Motion

• We are now going to investigate the effect of changing the mass of an object being accelerated by an unbalanced force.

Newton’s Second Law of Motion

experiment 2• A light gate is used to measure acceleration of a trolley.• The hanging mass is kept constant therefore there is an unbalanced force.

• Note: only the mass of the trolley is changed.

Results

Mass (kg)

a (m/s2)

0.20.40.6

Newton’s Second Law of Motion

Conclusion: As the mass of a body increases the acceleration decreases.

In other words a ~ 1m

Defining Newton’s Second Law of Motion

• Newton’s Second Law of Motion states that the acceleration of an object is directly proportional to the unbalanced force acting on it,

• e.g. a ~ F• and inversely proportional to the mass of

an object.• e.g. a ~ 1

m

Newton’s Second Law of Motion

• Combining the two proportions• a ~ F and a ~ 1

m• Gives us the following equation: a = F

m

• This is re-written as:Fun = ma

• Also known as Newton’s second Law

Problems involving several forces

1. In all situations apply: Fun = ma (work out the resultant unbalanced force)

2. Draw a sketch diagram for the whole system including masses and external forces.

3. Indicate the direction of acceleration on the object.

Example 1

• What is the mass of an object if an unbalanced force of 20 N produces an acceleration of 4 m/s2?

Example 2

• What is the acceleration of a 600 kg car, when the engine exerts a force of 1700 N, but the frictional force is 800 N?

Example 3

• A 6 kg block is dragged along with a horizontal force of 16 N.

• If the block accelerates at 2 m/s2, what is the force of friction acting on the block?

Lesson 4• Carry out calculations involving the

relationship between work done, unbalanced force and distance / displacement.

Work Done• Work done is a measure of the energy

transferred.• It has the symbol Ew and is measured in

Joules (J).• The work done to any object depends on

the “size of the force” being applied and the “distance” it is being moved along.Work done = Force x distanceEw = F d

Example 1Ew = ?F = 600 Nd = 4 m

Example 2Ew = 15 MJ F = ?d = 50 km

Example 3Ew = 6000 JF = 25 Nd = ?

2010

2008 Qu: 23

Lesson 5• Describe the difference between mass and

weight.• Investigate the relationship between mass

and weight.• State what is meant by gravitational field

strength.• Carry out calculations involving the

relationship between weight, mass and gravitational field strength during interplanetary rocket flight.

Mass and Weight• The mass of an object is the

quantity of matter it contains.• It is measured in kilograms (kg).• The weight of an object is a force

caused by the pull of the Earth on an object.

• It is measured in Newtons (N).

What to do• Collect a 1-10N / 1-15N / 1-20N Newton

balance and a set of slotted 100g masses.• Copy & complete the table below:

Mass (kg) Weight (N) Ratio of weight mass

0.2

0.4

0.6

Your mass =

Relationship Between Mass and Weight

• We now have an equation linking weight, mass and g:

• Weight = gmass

• This can be re-written as:• weight = mass x g. Or in symbol form:

W = m g

What is meant by gravitational field strength?

• The ratio of weight / mass is known as the gravitational field strength.

• This is represented by the symbol ‘g’.

• The value of g on Earth is usually taken as 10 N/kg.

• g is also known as the ‘weight per unit mass’.

Example• Copy and complete the table:

Object on Earth

(g=10N/kg)

Mass (kg) Weight (N)

Brick 3Concrete

block100

Bag of cement

500

Tonne of sand

1000

Gravitational Field Strength

• The value of ‘g’ varies as we move from one body to another in our solar system.

• The bigger the body, the bigger the value of g.• Our mass would remain the same and never

change if we went to these different bodies, however, our weight does.

• We can work out our weight (N) on different bodies using the equation W = mg as long as we have a value for ‘g’ for that body.

Body g (N/kg) Your Weight in N (W = mg)

Mercury 4Venus 9Earth 10Mars 4

Jupiter 26Saturn 11Uranus 12

Neptune 12Pluto 4Moon 1.6Sun 270

Lesson 6• Use Newton’s second law and apply

it to space travel including rocket launch and landing.

Newton’s 2nd Law (Fun = ma) and space travel

• When using Fun = ma in space travel we follow the same rules as before:

1. In all situations apply: Fun = ma (work out the resultant unbalanced force)

2. Draw a sketch diagram for the whole system including masses and external forces.

3. Indicate the direction of acceleration on the object.

However, we now must consider gravity and the force of weight. In other words, apply W = mg acting down on the object

Example

• What is the acceleration of a 700 kg helicopter, when the engine exerts a force of 9900 N vertically upwards, but the frictional force is 800 N?

• Fun = Fup – Ffr – W• W = mg = 700 x 10 =

7000N• Fun = 9900 – 800 -

7000 = 2000 N• a = Fun

m= 2100 700= 3 m/s2

2006

2003 Qu: 22 (1 of 2)

2003 Qu: 22 (2 of 2)

Lesson 7• State Newton’s third law.• Apply Newton’s third law to explain

motion resulting from a ‘reaction force’.

• Use of Newton’s laws to explain free-fall and terminal velocity.

Newton’s Third Law

• If object A exerts a force on object B then object B exerts an equal but opposite force on object A.

• In other words:‘action and reaction are equal and opposite’.

Lesson 7

Lesson 7

Balanced Forces: Parachute jump

• When a skydiver jumps from a plane he will only accelerate for a short while.

• The air friction rapidly increases until it equals the skydiver’s weight.

• The skydiver will then fall with a uniform velocity called terminal velocity.

Balanced Forces: Parachute jump

• Terminal velocity is the constant velocity reached by an object once it is no longer accelerating.

Sky diving

• What not to do!

2004