Monument & Energy│Please Help!

Question

A 2 kg ball moves to the right at 4 m/s towards a 4 kg ball that is at rest. After the collision, the 2 kg ball is at rest. What is the velocity of the 4 kg ball after the collision?

1. 2 m/s to the right
2. 4 m/s to the left
3. 4 m/s to the right
4. 2 m/s to the left

Answer 1

before: total momentum = m₁v₁ + m₂v₂ = (2)(4) + (4)(0) = 8

after: total momentum = m₁v₁_f + m₂v₂_f = (2)(0) + (4)(v₂_f) = 4v₂_f = 4v_f

Conservation of momentum: m₁v₁ + m₂v₂ = m₁v₁_f + m₂v₂_f

_So 4v_f = 8

v_f = 8/4 = 2 m/s

Answer

2 m/s to the right