MCQs / Physics / Momentum and Collisions

Momentum and Collisions MCQs — Physics

47 multiple-choice questions on Momentum and Collisions from Physics, each with the correct answer and, where available, a teacher's explanation. Read through them to revise the chapter, then practise the same questions one by one to test yourself.

1. A very heavy ball hits a light ball at rest elastically. The light ball moves off at about:

  1. zero
  2. half the speed
  3. the same speed
  4. twice the heavy ball's speed

Answer: D) twice the heavy ball's speed

Explanation: v2 approx 2u1 when m1 gg m2. SLO P-11-B-12 · Application

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2. A 4 kg rifle fires a 0.02 kg bullet at 400 m/s. The recoil speed of the rifle is:

  1. 1 m/s
  2. 4 m/s
  3. 2 m/s
  4. 20 m/s

Answer: C) 2 m/s

Explanation: Total momentum stays zero: MV = mv Rightarrow V = 0.02 4004 = 2 m/s (backwards). SLO P-11-B-11 · Application

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3. A 5 kg ball moving at 3 m/s makes a head-on elastic collision with a 5 kg ball at rest. The velocities after the collision are:

  1. v1 = 1.5 m/s, v2 = 1.5 m/s
  2. v1 = 3 m/s, v2 = 0 m/s
  3. v1 = 3 m/s, v2 = 1.5 m/s
  4. v1 = 0 m/s, v2 = 3 m/s

Answer: D) v1 = 0 m/s, v2 = 3 m/s

Explanation: v1 = m1 - m2m1 + m2u1 = 0 m/s and v2 = 2m1m1 + m2u1 = 3 m/s. Check: momentum and kinetic energy are both conserved. SLO P-11-B-12 · Application

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4. For a closed system, which statement is always true?

  1. Total momentum before = total momentum after
  2. Total KE before = total KE after
  3. Total speed is constant
  4. Each body keeps its momentum

Answer: A) Total momentum before = total momentum after

Explanation: Momentum conservation is exact for a closed system. SLO P-11-B-13 · Application

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5. A bullet embeds in a wooden block. Which is conserved?

  1. momentum only
  2. neither
  3. both momentum and kinetic energy
  4. kinetic energy only

Answer: A) momentum only

Explanation: It is a perfectly inelastic collision. SLO P-11-B-13 · Application

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6. A 2 kg trolley at 3 m/s couples with a 1 kg trolley at rest. The fraction of kinetic energy kept is:

  1. 13
  2. 1
  3. 12
  4. 23

Answer: D) 23

Explanation: v = 2 m/s; KE: 9 J to 6 J, i.e. 23. SLO P-11-B-13 · Application

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7. A 2 kg object at 6 m/s hits a 4 kg object at rest and they stick together. The kinetic energy lost is:

  1. 36 J
  2. 24 J
  3. 12 J
  4. 48 J

Answer: B) 24 J

Explanation: v = 2 m/s. KE before = 36 J, after = 12 J, so 24 J is converted to heat, sound and deformation. SLO P-11-B-13 · Application

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8. A 1000 kg object at 20 m/s hits a 1000 kg object at rest and they stick together. The kinetic energy lost is:

  1. 50,000 J
  2. 2 10^5 J
  3. 1 10^5 J
  4. 3 10^5 J

Answer: C) 1 10^5 J

Explanation: v = 10 m/s. KE before = 2 10^5 J, after = 1 10^5 J, so 1 10^5 J is converted to heat, sound and deformation. SLO P-11-B-13 · Application

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9. A light ball hits a very heavy ball at rest elastically. The light ball:

  1. sticks to the heavy ball
  2. rebounds with almost the same speed
  3. stops
  4. moves on with the same speed

Answer: B) rebounds with almost the same speed

Explanation: v1 approx -u1 when m2 gg m1. SLO P-11-B-12 · Application

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10. Two ice skaters at rest push off each other. Afterwards their total momentum is:

  1. equal to the heavier one's momentum
  2. doubled
  3. equal to the sum of their speeds
  4. zero

Answer: D) zero

Explanation: Momentum before = 0, so momentum after = 0. SLO P-11-B-11 · Application

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11. In a one-dimensional elastic collision, the relative speed of approach equals:

  1. twice the relative speed of separation
  2. zero
  3. the relative speed of separation
  4. the sum of the speeds

Answer: C) the relative speed of separation

Explanation: u1 - u2 = v2 - v1. SLO P-11-B-12 · Application

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12. In a car crash, crumple zones reduce injury because they:

  1. stop momentum being conserved
  2. increase the car's mass
  3. increase the collision time, reducing the force
  4. reduce the change in momentum

Answer: C) increase the collision time, reducing the force

Explanation: F = Delta p/Delta t; same Delta p over a longer time. SLO P-11-B-11 · Application

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13. A 1 kg ball at 4 m/s collides elastically with a 1 kg ball at rest. The kinetic energy of the second ball afterwards is:

  1. 4 J
  2. 8 J
  3. 16 J
  4. 2 J

Answer: B) 8 J

Explanation: It takes all the velocity: 12(1)(4)^2 = 8 J. SLO P-11-B-12 · Application

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14. A 3 kg body moving at 5 m/s collides with a 2 kg body moving at -2 m/s. They stick together. Their common velocity is:

  1. 4.4 m/s
  2. -2.2 m/s
  3. 2.2 m/s
  4. 1.1 m/s

Answer: C) 2.2 m/s

Explanation: m1u1 + m2u2 = (m1 + m2)v Rightarrow v = 3(5) + 2(-2)5 = 2.2 m/s. SLO P-11-B-11 · Application

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15. A 5 kg rifle fires a 0.05 kg bullet at 300 m/s. The recoil speed of the rifle is:

  1. 3 m/s
  2. 30 m/s
  3. 6 m/s
  4. 1.5 m/s

Answer: A) 3 m/s

Explanation: Total momentum stays zero: MV = mv Rightarrow V = 0.05 3005 = 3 m/s (backwards). SLO P-11-B-11 · Application

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16. A 2 kg ball moving at 10 m/s makes a head-on elastic collision with a 6 kg ball at rest. The velocities after the collision are:

  1. v1 = -5 m/s, v2 = 5 m/s
  2. v1 = 0 m/s, v2 = 3.33 m/s
  3. v1 = 2.5 m/s, v2 = 2.5 m/s
  4. v1 = 5 m/s, v2 = -5 m/s

Answer: A) v1 = -5 m/s, v2 = 5 m/s

Explanation: v1 = m1 - m2m1 + m2u1 = -5 m/s and v2 = 2m1m1 + m2u1 = 5 m/s. Check: momentum and kinetic energy are both conserved. SLO P-11-B-12 · Application

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17. Momentum is conserved in a collision in:

  1. neither
  2. inelastic collisions only
  3. elastic collisions only
  4. both elastic and inelastic collisions

Answer: D) both elastic and inelastic collisions

Explanation: Only kinetic energy is lost in inelastic collisions. SLO P-11-B-11 · Application

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18. A rocket ejects 2 kg of gas per second at 3000 m/s. The thrust on the rocket is:

  1. 600 N
  2. 1500 N
  3. 3000 N
  4. 6000 N

Answer: D) 6000 N

Explanation: Thrust = Delta mDelta tv = 2 3000 = 6000 N. SLO P-11-B-11 · Application

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19. A car crash in which the cars crumple is:

  1. an elastic collision
  2. one where momentum is lost
  3. an inelastic collision
  4. not a collision

Answer: C) an inelastic collision

Explanation: Energy goes into deforming metal. SLO P-11-B-13 · Application

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20. A 1 kg ball moving at 8 m/s makes a head-on elastic collision with a 3 kg ball at rest. The velocities after the collision are:

  1. v1 = 4 m/s, v2 = -4 m/s
  2. v1 = 2 m/s, v2 = 2 m/s
  3. v1 = -4 m/s, v2 = 4 m/s
  4. v1 = 0 m/s, v2 = 2.67 m/s

Answer: C) v1 = -4 m/s, v2 = 4 m/s

Explanation: v1 = m1 - m2m1 + m2u1 = -4 m/s and v2 = 2m1m1 + m2u1 = 4 m/s. Check: momentum and kinetic energy are both conserved. SLO P-11-B-12 · Application

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21. A 4 kg ball moving at 6 m/s makes a head-on elastic collision with a 1 kg ball at rest. The velocities after the collision are:

  1. v1 = 0 m/s, v2 = 24 m/s
  2. v1 = 4.8 m/s, v2 = 4.8 m/s
  3. v1 = 9.6 m/s, v2 = 3.6 m/s
  4. v1 = 3.6 m/s, v2 = 9.6 m/s

Answer: D) v1 = 3.6 m/s, v2 = 9.6 m/s

Explanation: v1 = m1 - m2m1 + m2u1 = 3.6 m/s and v2 = 2m1m1 + m2u1 = 9.6 m/s. Check: momentum and kinetic energy are both conserved. SLO P-11-B-12 · Application

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22. In a karate chop the hand exerts an average force of 2000 N for 0.005 s. The impulse given is:

  1. 10 N s
  2. 1 N s
  3. 100 N s
  4. 20 N s

Answer: A) 10 N s

Explanation: Impulse = FDelta t = 2000 0.005 = 10 N s = change in momentum. SLO P-11-B-11 · Application

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23. Snooker balls of equal mass: a moving ball hits a stationary ball head-on and stops. This shows the collision is:

  1. explosive
  2. (nearly) elastic
  3. perfectly inelastic
  4. impossible

Answer: B) (nearly) elastic

Explanation: Equal masses exchange velocities in an elastic collision. SLO P-11-B-12 · Application

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24. In an elastic collision:

  1. both momentum and kinetic energy are conserved
  2. neither is conserved
  3. only momentum is conserved
  4. only kinetic energy is conserved

Answer: A) both momentum and kinetic energy are conserved

Explanation: That is the definition of elastic. SLO P-11-B-12 · Application

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25. Momentum is always conserved in a collision of a closed system because:

  1. gravity acts
  2. the bodies are elastic
  3. kinetic energy is conserved
  4. the forces between the bodies are equal and opposite (Newton's third law)

Answer: D) the forces between the bodies are equal and opposite (Newton's third law)

Explanation: Internal impulses cancel. SLO P-11-B-13 · Application

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