AP Physics Physics: Momentum, Impulse & Collisions 2 — Questions and Answers
Question 1: The law of conservation of momentum applies to a system when:
- All forces within the system are conservative
- The net external force on the system is zero (Correct answer)
- All objects in the system have the same mass
- All objects move along the same line
Correct answer: The net external force on the system is zero
Momentum is conserved when the net external force on the system is zero, so the total momentum remains constant.
Question 2: In a perfectly inelastic collision:
- Kinetic energy is fully conserved
- The objects stick together after the collision (Correct answer)
- Momentum is not conserved
- The objects bounce off each other with no energy loss
Correct answer: The objects stick together after the collision
In a perfectly inelastic collision, the objects stick together and move as one unit, which represents the maximum possible loss of kinetic energy.
Question 3: Two equal-mass objects collide elastically in one dimension. Object 1 moves at v; object 2 is at rest. After the collision:
- Both objects move at v/2
- Object 1 continues at v; object 2 remains at rest
- Object 1 stops; object 2 moves at v (Correct answer)
- Both objects move in opposite directions
Correct answer: Object 1 stops; object 2 moves at v
In a 1D elastic collision between equal masses, the moving object stops and the stationary object moves with the original velocity.
Question 4: A 3 kg cart moving at 6 m/s collides with a stationary 3 kg cart. They stick together. What is their combined final velocity?
- 6 m/s
- 3 m/s (Correct answer)
- 1.5 m/s
- 0 m/s
Correct answer: 3 m/s
p_i = 3 × 6 = 18 kg·m/s; p_f = 6v → v = 18/6 = 3 m/s.
Question 5: In an elastic collision, which quantities are conserved?
- Momentum only
- Kinetic energy only
- Both total momentum and total kinetic energy (Correct answer)
- Total mechanical energy including potential energy
Correct answer: Both total momentum and total kinetic energy
Elastic collisions are defined by conservation of both total momentum and total kinetic energy.
Question 6: A 5 kg object moving at 4 m/s collides with a 3 kg object at rest. They move together after the collision. What is their final speed?
- 2.5 m/s (Correct answer)
- 4.0 m/s
- 1.5 m/s
- 3.0 m/s
Correct answer: 2.5 m/s
p_i = 5 × 4 = 20 kg·m/s; (5+3)v = 20 → v = 20/8 = 2.5 m/s.
Question 7: A 2 kg ball at 8 m/s collides head-on with a 4 kg ball at 2 m/s in the opposite direction. They stick together. What is the final velocity? (Take the 2 kg ball's direction as positive.)
- 4.0 m/s in the positive direction
- 0 m/s
- 1.33 m/s in the positive direction (Correct answer)
- 2.0 m/s in the negative direction
Correct answer: 1.33 m/s in the positive direction
p_i = 2(8) + 4(−2) = 16 − 8 = 8 kg·m/s; v = 8/(2+4) = 8/6 ≈ 1.33 m/s in the positive direction.
AP Physics Physics: Momentum, Impulse & Collisions 2
The law of conservation of momentum applies to a system when: