MCQs / Physics / Momentum and Collisions

Momentum and Collisions MCQs — Physics (page 2)

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.

26. A 2 kg ball moving at 5 m/s makes a head-on elastic collision with a 2 kg ball at rest. The velocities after the collision are:

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

Answer: B) v1 = 0 m/s, v2 = 5 m/s

Explanation: v1 = m1 - m2m1 + m2u1 = 0 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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27. When two equal masses collide head-on elastically (one at rest), they:

  1. exchange velocities
  2. both stop
  3. stick together
  4. move together at half speed

Answer: A) exchange velocities

Explanation: The moving one stops and the other moves off with its velocity. SLO P-11-B-12 · Application

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28. In a perfectly elastic collision, the total kinetic energy after the collision is:

  1. less than before
  2. zero
  3. equal to that before
  4. greater than before

Answer: C) equal to that before

Explanation: No energy is converted to heat or sound. SLO P-11-B-12 · Application

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29. Kinetic energy can change in a collision even though momentum doesn't, because:

  1. momentum is a scalar
  2. momentum depends on v^2
  3. kinetic energy is a scalar that can be converted to other forms, while momentum is a vector with no other forms
  4. kinetic energy is always conserved

Answer: C) kinetic energy is a scalar that can be converted to other forms, while momentum is a vector with no other forms

Explanation: Momentum has no 'other form' to convert into. SLO P-11-B-13 · Application

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30. Which collision is closest to perfectly elastic?

  1. a bullet embedding in a block
  2. collisions between gas molecules
  3. a ball of clay hitting a wall
  4. a car crash

Answer: B) collisions between gas molecules

Explanation: Ideal gas molecules collide elastically. SLO P-11-B-12 · Application

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31. The law of conservation of momentum applies when:

  1. kinetic energy is conserved
  2. friction is large
  3. no external force acts on the system
  4. the bodies are at rest

Answer: C) no external force acts on the system

Explanation: Internal forces cancel in pairs (Newton's third law). SLO P-11-B-11 · Application

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32. In a karate chop the hand exerts an average force of 500 N for 0.01 s. The impulse given is:

  1. 0.5 N s
  2. 50 N s
  3. 5 N s
  4. 10 N s

Answer: C) 5 N s

Explanation: Impulse = FDelta t = 500 0.01 = 5 N s = change in momentum. SLO P-11-B-11 · Application

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33. Two balls approach each other at 3 m/s and 5 m/s and collide elastically. Their speed of separation is:

  1. 15 m/s
  2. 4 m/s
  3. 2 m/s
  4. 8 m/s

Answer: D) 8 m/s

Explanation: Relative speed of approach = 3 + 5 = 8 m/s = relative speed of separation. SLO P-11-B-12 · Application

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34. A 3 kg object at 8 m/s hits a 1 kg object at rest and they stick together. The kinetic energy lost is:

  1. 24 J
  2. 96 J
  3. 12 J
  4. 48 J

Answer: A) 24 J

Explanation: v = 6 m/s. KE before = 96 J, after = 72 J, so 24 J is converted to heat, sound and deformation. SLO P-11-B-13 · Application

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35. A perfectly inelastic collision is one in which:

  1. the bodies bounce apart
  2. kinetic energy is conserved
  3. the bodies stick together after colliding
  4. momentum is lost

Answer: C) the bodies stick together after colliding

Explanation: Maximum kinetic energy is lost. SLO P-11-B-13 · Application

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36. Two bodies of equal mass moving in opposite directions at equal speeds collide and stick. Afterwards:

  1. momentum doubles
  2. they move at the same speed
  3. kinetic energy is unchanged
  4. they are at rest and all kinetic energy is lost

Answer: D) they are at rest and all kinetic energy is lost

Explanation: Total momentum was zero, so final velocity is zero. SLO P-11-B-13 · Application

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37. A 1000 kg body moving at 20 m/s collides with a 1500 kg body moving at 0 m/s. They stick together. Their common velocity is:

  1. 4 m/s
  2. 8 m/s
  3. 16 m/s
  4. -8 m/s

Answer: B) 8 m/s

Explanation: m1u1 + m2u2 = (m1 + m2)v Rightarrow v = 1000(20) + 1500(0)2500 = 8 m/s. SLO P-11-B-11 · Application

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38. In a karate chop the hand exerts an average force of 50 N for 0.2 s. The impulse given is:

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

Answer: D) 10 N s

Explanation: Impulse = FDelta t = 50 0.2 = 10 N s = change in momentum. SLO P-11-B-11 · Application

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39. In an inelastic collision, the 'lost' kinetic energy becomes:

  1. extra momentum
  2. potential energy of the Earth
  3. heat, sound and work done in deforming the bodies
  4. nothing — it disappears

Answer: C) heat, sound and work done in deforming the bodies

Explanation: Energy is conserved overall but not as kinetic energy. SLO P-11-B-13 · Application

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40. A 1 kg object at 10 m/s hits a 1 kg object at rest and they stick together. The kinetic energy lost is:

  1. 26 J
  2. 25 J
  3. 12.5 J
  4. 50 J

Answer: B) 25 J

Explanation: v = 5 m/s. KE before = 50 J, after = 25 J, so 25 J is converted to heat, sound and deformation. SLO P-11-B-13 · Application

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41. When a ball dropped on the floor bounces back to a lower height:

  1. energy was destroyed
  2. gravity changed
  3. some kinetic energy was converted to heat and sound in the bounce
  4. momentum is not conserved for the ball–Earth system

Answer: C) some kinetic energy was converted to heat and sound in the bounce

Explanation: The bounce was not perfectly elastic. SLO P-11-B-13 · Application

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42. A ball of mass 0.5 kg moving east at 4 m/s collides with an identical ball at rest; afterwards the first ball moves at 1 m/s east. The second moves at:

  1. 5 m/s east
  2. 3 m/s east
  3. 4 m/s east
  4. 1 m/s west

Answer: B) 3 m/s east

Explanation: 0.5(4) = 0.5(1) + 0.5v Rightarrow v = 3 m/s. SLO P-11-B-11 · Application

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43. A 0.5 kg body moving at 8 m/s collides with a 1.5 kg body moving at 0 m/s. They stick together. Their common velocity is:

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

Answer: A) 2 m/s

Explanation: m1u1 + m2u2 = (m1 + m2)v Rightarrow v = 0.5(8) + 1.5(0)2.0 = 2 m/s. SLO P-11-B-11 · Application

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44. A 3 kg rifle fires a 0.01 kg bullet at 600 m/s. The recoil speed of the rifle is:

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

Answer: B) 2 m/s

Explanation: Total momentum stays zero: MV = mv Rightarrow V = 0.01 6003 = 2 m/s (backwards). SLO P-11-B-11 · Application

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45. A 0.15 kg cricket ball at 30 m/s is hit straight back at 20 m/s. The change in its momentum is:

  1. 1.5 kg m/s
  2. 4.5 kg m/s
  3. 7.5 kg m/s
  4. 3.0 kg m/s

Answer: C) 7.5 kg m/s

Explanation: Delta p = 0.15(20 - (-30)) = 7.5 kg m/s. SLO P-11-B-11 · Application

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46. In an explosion of a body at rest, the kinetic energy of the fragments:

  1. equals the momentum
  2. stays zero
  3. increases from zero, while total momentum stays zero
  4. decreases

Answer: C) increases from zero, while total momentum stays zero

Explanation: Chemical energy becomes kinetic energy. SLO P-11-B-13 · Application

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47. A 2 kg body moving at 6 m/s collides with a 4 kg body moving at 0 m/s. They stick together. Their common velocity is:

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

Answer: D) 2 m/s

Explanation: m1u1 + m2u2 = (m1 + m2)v Rightarrow v = 2(6) + 4(0)6 = 2 m/s. SLO P-11-B-11 · Application

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