Physics: Momentum, Impulse & Collisions Flashcards

The law of conservation of momentum applies to a system when:

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.

In a perfectly inelastic collision:

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.

Two equal-mass objects collide elastically in one dimension. Object 1 moves at v; object 2 is at rest. After the collision:

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.

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?

Answer: 3 m/s

p_i = 3 × 6 = 18 kg·m/s; p_f = 6v → v = 18/6 = 3 m/s.

In an elastic collision, which quantities are conserved?

Answer: Both total momentum and total kinetic energy

Elastic collisions are defined by conservation of both total momentum and total kinetic energy.

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?

Answer: 2.5 m/s

p_i = 5 × 4 = 20 kg·m/s; (5+3)v = 20 → v = 20/8 = 2.5 m/s.

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.)

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.