## Unit 3 Laws Of Motion Chapter 1 Newton Law Of Motion Multiple Choice Questions And Answers

**Question 1. A closed container filled with gas is moving horizontally with some acceleration. The pressure of the gas in the container (neglecting gravitational pull) is**

- Same throughout
- Comparatively less at the front
- Comparatively less at the back wall
- Comparatively less in the upper part

**Answer:** 2. Comparatively less at the front

**Question 2. Two masses, m and 2 m are connected by a string that passes over a frictionless pulley. When the mass 2m is released, the acceleration of the mass m upwards will be**

- g/3
- g/2
- g
- 2g

**Answer:** 1. g/3

**Question 3. The masses of blocks A and B are 2 kg and 3 kg respectively. The blocks are kept at rest on a frictionless horizontal table. When a horizontal force of 10 N is applied to A, the force applied on B by A will be**

- 4N
- 6N
- 8N
- 10N

**Answer:** 2. 6N

**Question 4. A person of mass M is at a height h from a floor in a place free from gravitational force. The person throws a ball of mass m, downward with a velocity u. Distance of the person from the floor, when the ball touches the floor, will be**

- \(h\left(1+\frac{m}{M}\right)\)
- \(h\left(2-\frac{m}{M}\right)\)
- 2h
- \(5 h\left(4+\frac{m}{2 M}\right)\)

**Answer:** 1. \(h\left(1+\frac{m}{M}\right)\)

**Question 5. A block is released from the top of an inclined plane of inclination θ and height h. The time required to reach the foot of the inclined plane is**

- \(\sqrt{\frac{2 h}{g}}\)
- \(\sin \theta \sqrt{\frac{2 h}{g}}\)
- \(\frac{1}{\sin \theta} \sqrt{\frac{2 h}{g}}\)
- \(\frac{1}{\cos \theta} \sqrt{\frac{2 h}{g}}\)

**Answer:** 3. \(\frac{1}{\sin \theta} \sqrt{\frac{2 h}{g}}\)

**Question 6. A plumb line is hanging from the roof of a car. When the car moves with acceleration a, the angle that the plumb line makes with the vertical is**

- \(\tan ^{-1} \frac{a}{g}\)
- \(\tan ^{-1} \underset{a}{g}\)
- \(\cos ^{-1} \frac{a}{g}\)
- \(\sin ^{-1} \frac{g}{a}\)

**Answer:** 1. \(\tan ^{-1} \frac{a}{g}\)

**Question 7. Consider an elevator moving downwards with an acceleration a, the force exerted by a passenger of mass m in the floor of the elevator is**

- ma
- ma-mg
- mg- ma
- mg+ ma

**Answer:** 3. mg- ma

**Question 8. A monkey is descending from the branch of a tree with a constant acceleration. If the breaking strength of the branch is 75% of the weight of the monkey, the minimum acceleration with which the monkey can slide down without breaking the branch is**

- g
- 3g/4
- g/2
- g/4

**Answer:** 4. g/4

**Question 9. A car moving with a speed of 50 km/hr can be stopped by brakes, over a distance of 6 m. If the same car is moving at a speed of 100 km/hr, the stopping distance is**

- 12 m
- 18 m
- 6 m
- 24 m

**Answer:** 4. 24 m

**Question 10. The x and y coordinates of a particle at any time t are given by x = 7t + 14t² and y = 5t, where x and y are in meters and t is in seconds. The acceleration of the particle at t = 5 s is**

- Zero
- 8 m · s
^{-2} - 20 m · s
^{-2} - 40 m · s
^{-2}

**Answer:** 2. 8 m · s^{-2}

**Question 11. A ball of mass 0.5 kg is moving with a velocity v of 2 m · s ^{-1}. It is subjected to a force of xN in 2 s. Because of this force, the ball moves with a velocity of 3 m · s^{-2}. The value of x is**

- 5 N
- 8.25 N
- 0.25 N
- 1N

**Answer:** 3. 0.25 N

**Question 12. A force F _{1} of 500 N is required to push a car of mass 1000 kg slowly at constant speed on a level road. If a force F_{2} of 1000 N is applied, the acceleration of the car will be**

- Zero
- 1.5 m · s
^{-2} - 1 m · s
^{-2} - 0.5 m · s
^{-2}

**Answer:** 4. 0.5 m · s^{-2}

**Question 13. A cricket ball of mass 0.5 kg, moving at 30 m · s ^{-1}, hits a bat perpendicularly and rebounds with a velocity of 20 m · s^{-1}. The impulse of the force exerted by the ball on the bat i**s

- 0.5 N · s
- 1.0 N · s
- 25 N · s
- 50 N · s

**Answer:** 3. 25 N · s

**Question 14. A rocket consumes fuel at the rate of 100 kg · s ^{-1}. Gas ejects out of it with a velocity of 5 x 10 m · s^{-1}. If the gravitational pull is neglected, the impulsive force experienced by the rocket is**

- 5 X10²N
- 5 X 10
^{4}N - 5 X10
^{6}N - 5 X10
^{8}N

**Answer:** 3. 5X10^{6}N

**Question 15. A ball of mass m is bowled at a velocity v to a batsman. The batsman hits the ball, deflecting its direction by an angle θ, without changing the magnitude of velocity. The impulse of the ball is**

- mv cosθ
- mv sinθ
- \(2 m v \cos ^2 \frac{\theta}{2}\)
- \(2 m v \cos \frac{\theta}{2}\)

**Answer:** 4. \(2 m v \cos \frac{\theta}{2}\)

**Question 16. A bird sits on a stretched telegraph wire. The increase in tension in the wire is**

- Equal to the weight of the bird
- More than the bird’s weight
- Less than the bird’s weight
- Zero

**Answer:** 2. More than the bird’s weight

**Question 17. A block of mass 10 kg is suspended by three strings as shown. The tension T _{2} is**

- 100 N
- \(\frac{100}{\sqrt{3}} \mathrm{~N}\)
- √3x 100 N
- 50√3N

**Answer:** 4. 50√3N

**Question 18. Two bodies of masses 5 kg and 3 kg respectively are connected to two ends of a light string passing over a horizontal frictionless pulley. The tension in the string is (g = 9.8 m/s²)**

- 60 N
- 36.75 N
- 73.50 N
- 18 N

**Answer:** 2. 36.75 N

**Question 19. If the elevator is moving upwards with a constant acceleration of 1 m/s², the tension in the string connected to block A of mass 6 kg would be (g = 10 m/s²)**

- 60 N
- 66 N
- 54 N
- 42 N

**Answer:** 2. 66 N

**Question 20. A man of weight w is in a lift that is moving up with an acceleration a. If acceleration due to gravity is g, the apparent weight of the man will be**

- \(w\left(1+\frac{a}{g}\right)\)
- \(w\left(1-\frac{a}{g}\right)\)
- w
- Zero

**Answer:** 1. \(w\left(1+\frac{a}{g}\right)\)

**Question 21. A thief steals a treasure box of weight w. He then jumps off a wall of height h with the box on his head. The weight felt on his head before he touches the ground is**

- 2 w
- w
- \(\frac{w}{2}\)
- Zero

**Answer:** 4. Zero

**Question 22. The working principle of a jet engine is based on the principle of**

- Conservation of mass
- Conservation of energy
- Conservation of linear momentum
- Conservation of angular momentum

**Answer:** 3. Conservation of linear momentum

**Question 23. A free particle of mass m was in motion along the x-axis on a horizontal x-y plane kept at a fixed height above the earth. On sudden explosion, the particle broke up into two pieces of masses \(\frac{m}{4}\) and \(\frac{3m}{4}\). After an interval of time, the position of the smaller fragment along the y-axis became y = 15 cm. At that moment, the position of the larger piece was**

- y = -5 cm
- y = + 20 cm
- y = + 5 cm
- y = -20 cm

**Answer: **1. y = -5 cm

**Question 24. A truck, carrying sand, moves with uniform velocity u on a smooth horizontal road. If Δm mass of sand falls in time Δt from the truck, then to maintain the speed u, the truck needs a force**

- \(\frac{\Delta m u}{\Delta t}\)
- \(\frac{\Delta m u}{2 \Delta t}\)
- \(\frac{\Delta m u^2}{\Delta t}\)
- Zero

**Answer:** 4. Zero

**Question 25. A few lead pellets, each of mass m, fall on a horizontal plane. Each pellet touches the plane with velocity u. If n number of pellets fall per second and none renounces, applied force on the horizontal plane will be**

- \(\frac{m u}{n}\)
- \(n m u\)
- \(\frac{n m}{u}\)
- \(\frac{m}{n u}\)

**Answer**: 2. \(n m u\)

**Question 26. A boy of mass m is standing on a wooden plank, of mass M and length l, floating on water. If the boy walks over the plank at a constant velocity from one end to the other, the displacement of the plank is**

- \(\frac{m l}{M}\)
- \(\frac{M l}{m}\)
- \(\frac{m l}{(M+m)}\)
- \(\frac{m l}{(M-m)}\)

**Answer:** 3. \(\frac{m l}{(M+m)}\)

**In this type of question, more than one options are correct.**

**Question 27. Suppose a body that is acted on by exactly two forces is accelerated. For this situation mark out the incorrect statements.**

- The body can not move with constant speed
- The velocity can never be zero
- The resultant of two forces cannot be zero
- The two forces must act in the same line

**Answer:**

1. The body can not move with constant speed

2. The velocity can never be zero

4. The two forces must act in the same line

**Question 28. Which of the following statements can be explained by Newton’s second law of motion?**

- To stop a heavy body (say truck), greater force is needed than to stop a light body (say motorcycle) in the same time if they are moving at same speed
- For a body of given mass, the greater the speed, the greater the opposing force needed to stop the body in a particular time duration
- To change the momentum of a body by given value, the force required is independent of time
- The same force acting on two different bodies for same time causes the same change in momentum for the different bodies

**Answer:**

1. To stop a heavy body (say truck), greater force is needed than to stop a light body (say motorcycle) in the same time if they are moving with same speed

2. For a body of given mass, the greater the speed, the greater the opposing force needed to stop the body in a particular time duration

4. The same force acting on two different bodies for the same time causes the same change in momentum for the different bodies