Question

A 40 kg boy is running at a speed of 3.0 m/s, when he jumps on a stationary skateboard. If the system, consisting of the boy and the skateboard, begins to roll at a speed of 2.5 m/s, what is the mass of the skateboard?

Answer 1

8 kg

Explanation:

Momentum(P)=mass(m) times velocity(v)

The total momentum of a system will not change (P initial = P final), so momentum of this system is:
`m(boy) * v(boy) + m(board) * v(board) = [m(boy) + m(board)] * v(boy and board)`

`40 kg*3.0m/s +m(board)*0m/s=[40 kg+m(board)]*2.5 m/s`

`120 kgm/s=[40 kg+m(board)]*2.5 m/s`

`(120kgm/s)/2.5 m/s=[40 kg+m(board)]`

`48kg=[40 kg+m(board)]`

`m(board)=48kg-40kg=8kg`