PHYSICS IIT-JEE Laws of Motion | AskIITian

8/10/2019 PHYSICS IIT-JEE Laws of Motion | AskIITian

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4 Note the following situations:

In which case will the magnitude of the normal force on the block be equal to + ?(a)Case 1 only

(b)

Case 2 only

(c)Both cases 1 and 2

(d)

Both cases 2 and 3

5 Two identical blocks are pulled along a rough surface as suggested in the figure. Which one of the

following statements is false?

(a)

The coefficient of kinetic friction is the same in each case.

(b)

A force of the same magnitude is needed to keep each block moving.

(c)

A force of the same magnitude was required to start each block moving.

(d)

The magnitude of the force of kinetic friction is greater for the block on the right.

6 A crate rests on the flatbed of a truck that is initially travelling at 15 m/s on a level road. The

driver applies the brakes and the truck is brought to a halt in a distance of 38 m. If the

deceleration of the truck is constant, what is the minimum coefficient of friction between the crate

and the truck that is required to keep the crate from sliding?

(a)0.20

(b)0.30

(c)

0.39

(d)0.59


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7

A 2 N rock slides on a frictionless inclined plane. Which one of the following statements is true

concerning the normal force that the plane exerts on the rock?

(a)The normal force is 0 N

(b)

The normal force is 2 N

(c)

The normal force is less than 2 N but more than 0 N

(d)The normal force is greater than 2 N

8

In an experiment with a block of wood on an inclined plane, with dimensions as shown in the

figure, the following observations are made:

(1) If the block is placed on the inclined plane, it remains there at rest.

(2) If the block is given a small push, it will accelerate towards the bottom of the incline without any

further pushing

Which is the best conclusion that can be drawn from these observations?

(a)The coefficient of kinetic friction must be negative.

(b)

Both coefficients of friction must be less than 0.25

(c)The coefficients of friction must be greater than 0.25

(d)The coefficient of static friction must be less than the coefficient of kinetic friction

9

A rock is suspended from a string, and it moves downward at a constant speed. Which one of the

following statements is true concerning the tension in the string if the air resistance is ignored?

(a)The tension is zero N

(b)The tension is equal to the weight of the rock

(c)The tension is less than the weight of the rock.

(d)

The tension is greater than the weight of the rock.


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10

In a tug of war, each man on a 5 man team pulls with an average force of 500 N. What is thetension in the centre of the rope?

(a)0 N

(b)100 N

(c)500 N

(d)2500 N

11

Under what condition(s) will an object be in equilibrium?

(a)If the object is either at rest or moving with a constant velocity, it is in equilibrium.

(b)If the object is either moving with constant velocity or with constant acceleration, it is in

equilibrium.

(c)

Only if the object is at rest can it be in equilibrium.(d)Only if the object is moving with constant velocity can it be in equilibrium.

12

A 4 kg block is connected by means of a massless rope to a 2 kg block as shown in the figure.Complete the following statement: If the 4 kg block is to begin sliding, the coefficient of static

friction between the 4 kg block and the surface must be

(a)

Less than 0.

(b)Greater than 2.

(c)Greater than 1 but less than 2.

(d)Greater than 0.5 but less than 1.

13 A small plane climbs with a constant velocity of 250 m/s at an angle of 28 0with respect to the

horizontal. Which statement is true concerning the magnitude of the net force on the plane?

(a)It is equal to the component of the weight of the plane in the direction of motion

(b)It is equal to the weight of the plane.

(c)It is equal to the magnitude of the force of air resistance.

(d)

It is less than the weight of the plane but greater than 0 N.


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14

A muscle builder holds the ends of a massless rope. At the centre of the rope, a 15 kg ball is hung

as shown. What is the tension in the rope if the angle in the drawing is 4. 50?

(a)

1900 N

(b)940 N

(c)470 N

(d)230 N

15 A block of mass M is hung by ropes as shown. The system is in equilibrium. The point O

represents the knot, the junction of the three ropes. Which of the following statements is true

with regard to the magnitudes of the three forces in equilibrium?

(a)1 = 2 = 3(b)1 > 3(c)2 < 3(d)1 = 2 = 32

16 A 20 kg crate is suspended from a fixed beam by two vertical ropes. What is the approximate

tension in each rope?

(a)

10 N

(b)40 N

(c)

100 N(d)

200 N


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17

Two sleds are hooked together in tandem as shown in the figure. The front sled is twice as

massive as the rear sled. The sleds are pulled along a frictionless surface by an applied force F.

The tension in the rope between the sleds is T. Determine the ratio of the magnitudes of the two

forces

(a)0.25

(b)0.33

(c)

0.50

(d)0.67

18 A woman stands on a bathroom scale in an elevator that is not moving. The scale reads 500 N. The

elevator then moves downward at a constant velocity of 5 m/s. What does the scale read when

the elevator descends with constant velocity?

(a)100 N

(b)

250 N

(c)500 N

(d)600 N

19 A rope connects boats A and B. Boat A starts from rest and accelerates to a speed of 9.5 m/s in a

time t = 47 sec. The mass of the boat B is 540 kg. Assuming a constant frictional force of 230 Nacting on the boat B, what is the magnitude of the tension in the rope that connects the boats

during the time that boat A is accelerating?

(a)340 N

(b)

1270 N

(c)230 N

(d)860 N

20 A certain crane can provide a maximum lifting force of 25,000 N. It hoists a 2000 kg load starting

at the ground level by applying the maximum force for a 2 second interval; then it applies just

sufficient force to keep the load moving upwards at a constant speed. Approximately how longdoes it take to raise the load from ground level to a height of 30 m?

(a)2 sec

(b)5 sec

(c)

8 sec

(d)9 sec


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21

A 10 kg block is pushed against a vertical wall by a horizontal force of 100 N as shown. The

coefficient of static friction

between the block and the wall is 0.60 and the coefficient of kinetic

friction is 0.40. Which one of the following statements is true if the block is initially at rest?

(a)

The total force exerted on the block by the wall is directed horizontally.

(b)The block slides down the wall with an acceleration of magnitude 3.8 m/s 2.

(c)The block will slide down the wall because the force of static friction can no longer be larger than

60N

(d)The block will remain at rest because the coefficient of static friction is greater than the coefficient

of kinetic friction.

22

A 20 kg package is dropped from a high tower in still air and is tracked by a radar system. When

the package is 25 m above the ground, the radar tracking indicates that its acceleration is 7 m/s 2.

Determine the force of air resistance on the package.

(a)56 N

(b)28 N

(c)340 N

(d)

196 N

23

A 4 kg block and a 2 kg block can move on the horizontal frictionless surface. The blocks are

accelerated by a 12 N force that pushes the larger block against the smaller one. Determine the

force that the 2 kg block exerts on the 4 kg block.

(a)

4 N

(b)12 N

(c)

0 N(d)8 N


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24

A system of two cables supports a 150-N weight as shown. What is the tension in the right handcable?

(a) 87 N

(b) 150 N

(c) 170 N

(d) 300 N

25

Three spring scales are attached along a straight line as shown. The scale on the left is attached to

a wall. A force of 15 N is applied to the scale at the right.

What is the reading on the middle scale?

(a)

45 N

(b)

10 N

(c)5 N

(d)15 N

26 At a playground, a child slides down a slide that makes an angle of 42with the horizontal

direction. The coefficient of kinetic friction for the child sliding on the slide is 0.2. What is the

magnitude of her acceleration during her sliding?

(a)4.7 m/s2

(b)5.1 m/s2

(c)

5.4 m/s2(d)

6.3 m/s2


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27

Which statement best explains why the weight of an object of mass m different on Mars than it is

on Earth?

(a)

The mass of Mars is different from that of Earth.

(b)The masses and radii of Mars and Earth are not the same.

(c)The mass m is further from the Earths center when it is on Mars.(d)The mass m will be different on Mars.

28 A 2 kg projectile is fired at an angle of 200. What is the magnitude of the force exerted on the

projectile when it is at the highest point in its trajectory? Neglect any effects of air resistance.

(a)

19.6 N

(b)14.7 N

(c)9.80 N

(d)

4.90 N

29 Two point masses m and M are separated by a distance d. If the separation d remains fixed and

the masses are increased to the values 3m and 3M respectively, how doesthe gravitational force

between them change?

(a)The force will be one third as great(b)The force will be one ninth as great(c)The force will be three times as great

(d)The force will be nine times as great

30 Two forces act on a 4.5 kg block resting on a frictionless surface as shown. What is the magnitudeof the horizontal acceleration of the block?

(a)

1.8 m/s2

(b)1.2 m/s2

(c)0.82 m/s2

(d)3.2 m/s2


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EXPLANATIONS

1

(c)

The magnitude of the normal reaction would be equal to the weight of the child which is the

reading shown by the scale.

2

(d)

The block is going to stop once the force of friction is equal to the retarding force.

Hence, using equations of motion, we get:

= 2

2

The retardation would be given by:

= 2010

= 2 /2

Hence, we get:

= 2

2 =62

2 2= 9

3 (d)

Since we are given that the net speed is equal to zero, so we can say that:

= 0Now, along the y axis, we get: = 0And along the x axis, we get:

+ = 0Now = = + = 299.8270 + 250 270 = 94 Hence, the normal reaction would be given by:

= + = 299.8270 + 250270 = 367

And thus, = = 0.26


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4 (a)

The normal reaction in each case would be as follows:

Case 1: The normal reaction would be given by:

= + Case 2: The normal reaction would be given by:

= Case 3: The normal reaction would be given by:

= 5 (d)

The coefficient of kinetic friction doesnt depend upon the amount of surface in contact. The amount of force of kinetic friction is greater for the block on the right as the length of contact

is more.

6 (b)

The acceleration produced while covering the distance of 38m would be given by:

= 2

2 =152

2 3 8=

225

76

Now, if the truck stops, then

=

=

225

76

9.8= 0.30

7 (c)

The normal reaction force is equal to = = 2. Hence the normal reaction is lessthan 2 N.


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8 (c)

The Normal reaction of the block would be given by = and the force acting along theincline would be given by . Hence, for the block to be at rest, the friction force should bemore than the downward force, i.e.

. Thus = 0.25.9 (b)

Since the rock is moving at a constant speed, so the net force acting would be equal to zero. Hence,

the tension is equal to the weight of the rock.

10

(a)

The tension in the centre of the rope will be equal to 0 N as on each side, the forces have to be

equal before any team wines the same.

11

(a)

When the object is either at rest of moving with an uniform velocity, then it is in equilibrium.

12

(d)

The acceleration of the blocks would be given by:

= 3

1 2

Hence, for the 4 kg block to start sliding, the acceleration should be positive and thus the

coefficient of friction must be between 0.5 and 1.

13

(a)

The weight of the plane is equal to the component of the force i.e. 28 = 14 (b)

We would have:

2 = Hence,

= 2 =

15 9.8

24.5 = 936.79 940


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15 (a)

Equating the horizontal forces, give us:

130 = 2 cos 30 1 = 2And equating the vertical components, give us:

1 + 2 sin30 = 3 + 21 12 = 1 = 3 +

16

(c)

The net tensions would be given by:

2

=

=

2

=2 0 1 0

2= 100

17 (b)

The net force would be equal to: = 3

Hence, = = 3 =

3

.

So, =

1

3= 0.33.

18

(b)

When the elevator is moving downwards with a constant velocity, the net weight is equal to the

reading of the scale.

19 (a)

The acceleration of the boat A would be equal to:

=9.5

47 = 0.2 /2

The net tension would be equal to:

= + = 230 + 540 0.2 = 338 340


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20 (c)

The net force acting on the load will be given by:

= + Hence, the acceleration would be given by:

= 25,0002,000

10 = 2.5 /2

the velocity achieved is equal to: = + = 0 + 2.5 2 = 5 /

Thus, the time required would be equal to:

= =30

5= 6

So, the total time taken would be equal to 8 sec.

21

(c)

The maximum frictional force is equal to 60 N which cannot balance the weight of the block which

is equal to 100 N.

22

(c)

The net force of air resistance would be given by:

= + = + = 201 0 + 7 = 340 23 (a)

The acceleration of the blocks due to the force would be equal to:

=

12

4+2= 2

Hence, the force exerted by the 2 kg block on the 4 kg block would be equal to

= 2 2 = 4


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24 (d)

The forces along horizontal direction would be equal to:

1 = 2 cos 30The forces along vertical direction would be equal to:

2 sin30 = 15025 (b)

The reading on the middle scale would be equal to 15 2

3= 10

26 (a)

The net acceleration of the child would be given by:

= 1 sin 42 =1

sin 42 = sin 42 = sin 42 = 100.6691 0.2 = 4.69 /2

27

(b)

The weight of an object depends on the acceleration due to gravity on the surface of the planet.

The acceleration due to gravity on any surface depends on the mass and the radius of that surface.

28 (a)

At the highest point of the trajectory, the velocity of the projectile along the vertical direction

would be zero. Hence, the net acceleration on the projectile would be equal to

= 9.8 /2Hence, the net force acting on the projectile would be equal to19

6 N.

29

(d)

The gravitational force is directly proportional to the product of the masses of the objects and

thus the new force will be nine times as great.

30 (a)

The net acceleration of the block in the horizontal direction would be

= 5.9 cos43 + 3.74.5

= 1.78 /2

PHYSICS IIT-JEE Laws of Motion | AskIITian

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