Free Linear Momentum MCQs with Answers

1. The SI unit of linear momentum is

• A. N s-1
• B. kg m s-1
• C. kg m s-2
• D. N m

Explanation: Momentum is mass times velocity, so its unit is kilogram metre per second. It is also equal to the newton second, since a newton is kg m s-2 and multiplying by a second gives kg m s-1. The unit kg m s-2 is the newton, a force, and the newton metre is the joule.

2. The second law can also be written as force equals

• A. the rate of change of momentum
• B. momentum multiplied by time
• C. mass divided by acceleration
• D. work divided by distance only

Explanation: F equals ma is the special case of this for constant mass; the momentum form is more general and handles situations such as a rocket losing mass as it burns fuel. It also leads directly to the impulse momentum theorem, that force multiplied by time equals change in momentum. That is why a longer collision time means a smaller force.

3. The impulse delivered to a body equals

• A. its change in kinetic energy
• B. its change in momentum
• C. the work done on it
• D. its mass times its velocity

Explanation: Impulse is force multiplied by the time for which it acts, and it equals the change in momentum, which is why airbags and crumple zones extend the collision time and so reduce the force on the occupants. A cricketer moving the hands back while catching does the same thing. Change in kinetic energy is related to work rather than to impulse.

4. A rocket moves forward in the vacuum of space because

• A. the exhaust gases push against the air behind it
• B. the momentum carried away by the exhaust gases is balanced by an equal forward momentum of the rocket
• C. gravity pulls it forward
• D. it becomes lighter as fuel burns

Explanation: Conservation of momentum, equivalently Newton's third law, means expelling mass backwards at speed drives the rocket forwards, and no surrounding medium is needed. This is why rockets work best in a vacuum, where there is no air resistance. Losing mass does help the acceleration but is not the source of the thrust.

5. The momentum of a body of mass 4 kg moving at 5 m per second is

• A. 20 kg m per second
• B. 0.8 kg m per second
• C. 50 kg m per second
• D. 9 kg m per second

Explanation: Momentum is mass multiplied by velocity, so 4 multiplied by 5 gives 20 kg m per second in the direction of motion. Its kinetic energy would be half times 4 times 25, that is 50 J, which is the trap in the third option. Momentum is a vector while kinetic energy is a scalar.

6. A cannon is placed on a smooth surface. When it fires a shell, the cannon moves backward. This recoil occurs due to:

• A. Law of conservation of energy
• B. Backward thrust of the gases
• C. Newton's third law of motion
• D. Newton's first law of motion

Explanation: The cannon pushes the shell forward and the shell pushes the cannon back with an equal and opposite force, which is the third law, and the same situation expressed as conservation of momentum gives the recoil velocity. Because the cannon is far more massive than the shell, its backward speed is correspondingly small. Energy is conserved too, but it does not by itself explain the direction of the motion.