Question

A 50kg boy jumps off the back of a 4 kg skateboard moving forward. Find the skateboard’s velocity immediately after the boy jumps, assuming that the skateboard’s initial velocity is 5 m/s and the boy’s velocity when jumping off the back is 2.5 m/s in the forward direction.

Answer 1

The mass of the boy and skateboard is there to throw you off. X and Y velocities are independent of each other, meaning that the only things you need are the velocity of the skateboard and the velocity of the boy. The total velocity of the boy is 7.5 m/s because the boy would still be moving with the 5 m/s that the skateboard has if the boy were to just jump straight up. Since the boy jumps forward with a velocity of 2.5 m/s, you would add them together.