Question

A 50 kg boy jumps off the back of a 4 kg skateboard. At the start, the boy and board are moving together at 5 m/s. The boy’s velocity after the jump has been reduced to 2.5 m/s, but in the same direction. Calculate the skateboard’s velocity immediately after the boy jumps

Answer 1

36.25 m/s.

Explanation:

The momentum of the board and the boy after the boy jumps is the same as the momentum when the boy was on the board. ( Conservation of Momentum Law).

Momentum = mass * velocity.

So MV (the momentum before the jump) = 54 * 5 = 270 kg m/s.

After it is 50 * 2.5 + 4V where V = velocity of the board immediately after the jump.

270 = 125 + 4V

4V = 145

V = 36.25 m/s.