Momentum + Collisions

A skateboard of mass 2kg is moving at 14m/s when it hits a ball of 200g, which is at rest. Immediately after the collision, the skateboard is moving at 12m/s in the same straight line, and in the same direction. What is the speed of the ball immediately after the collision?


I keep getting 20m/s, but the correct answer is 10m/s

Post your working out.

Momentum of skateboard before = 2kg * 14m/s = 28kg m/s
Momentum of ball before = 0.2kg * 0m/s = 0kg m/s
Momentum of skateboard after = 2kg * 12m/s = 24kg m/s

Momentum of ball after = 28kg m/s - 24 kg m/s = 4 kg m/s

Speed of ball after = (4kg m/s) / 0.2kg = 20m/s

Nothing wrong there. The given answer of 10 m/s is incorrect.

Note: 10 m/s cannot be correct, as it would imply that the skateboard had passed through the ball - which generally doesn't happen in this universe.