# Physics Questions and Answers – Laws of Motion – Conservation of Momentum

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This set of Physics Question Bank focuses on “Laws of Motion – Conservation of Momentum”.

1. Two bodies moving with constant velocities collide with each other. Which of the following quantities remain conserved?
a) Momentum
b) Speed
c) Force
d) Velocity

Explanation: Two bodies collide in the absence of any external force. In this case the momentum will remain constant because there is no force, hence, there is no change in momentum. The velocity and speed of the bodies might change after collision.

2. A ball is moving with a constant velocity. After some time, it collides with a wall. Which one of the following remains conserved except momentum?
a) Energy
b) Displacement
c) Power
d) Force

Explanation: Apart from the momentum, the energy also remains conserved during collision. As a matter of fact, the energy of a system often remains conserved.

3. A variable mass body with an initial mass of 5 kg is moving with a velocity of 5 m/s initially, after some time the velocity increases to 10 m/s, what is the new mass if the body is moving in the absence of any external force?
a) 2.5 Kg
b) 5 Kg
c) 7.5 Kg
d) 10 Kg

Explanation: The body is moving in the absence of any external force thus the momentum remains conserved. m1v1 = m2v2. v1 = 5 m/s, m1 = 5 Kg, v2 = 10 m/s. On solving, we get, m2 = 2.5 kg. Hence, the correct answer is 2.5 Kg.
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4. Two bodies of masses 2 Kg and 7 Kg are moving with velocities of 2 m/s and 7 m/s respectively. What is the total momentum of the system in Kg-m/s?
a) 50
b) 53
c) 28
d) 0

Explanation: Both the bodies are moving with constant velocity, hence both of them have momentum. The momentum of the system is the sum of the momenta of the bodies. Hence total momentum = 2 x 2 + 7 x 7 = 53 Kg-m/s.

5. What is the plural of the word ‘momentum’?
a) Momenta
b) Momentums
c) Moments
d) Momentuma

Explanation: The plural for momentum is momenta and not momentums. Momenta, being a plural, is usually used in cases where we need to address a more than one momentum. This situation can possibly arise in systems having more than one moving body.
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6. Two bodies are accelerating towards each other. After sometime they collide. Which one of the following statements is true about the system?
a) The total momentum remains conserved about the instant of collision
b) The total momentum remains conserved during collision
c) The total momentum does not remain conserved
d) The total momentum remains conserved

Explanation: The two bodies are accelerating and they collide. Since they are accelerating the total momentum does not remain conserved during the motion. But at the instant of collision, both the bodies have a constant velocity. Hence, just before and just after the collision, the total momentum will remain conserved.

7. When a body collides with a wall or the ground, what assumption do we make?
a) The mass of the body is not negligible
b) The body is stationary
c) The mass of the body is negligible as compared to the mass of the wall or the ground
d) The body is perfect

Explanation: When a body collides with a wall or the ground, we consider that the mass of the body is negligible as compared to the mass of the wall or the ground. We also make this assumption when something like a feather, which has a very small mass collides with a truck, which, comparatively, has a huge mass.

8. A variable mass body with an initial mass of 10 kg is moving with a velocity of 7 m/s initially, after some time the velocity increases to 14 m/s, what is the new mass if the body is moving in the absence of any external force?
a) 2.5 Kg
b) 5 Kg
c) 7.5 Kg
d) 10 Kg

Explanation: The body is moving in the absence of any external force thus the momentum remains conserved. m1v1 = m2v2. v1 = 7 m/s, m1 = 10 Kg, v2 = 14 m/s. On solving, we get, m2 = 5 kg. Hence, the correct answer is 5 Kg.

Sanfoundry Global Education & Learning Series – Physics – Class 11. 