What will be her speed and direction of motion after she throws the ball?

Question

A 60 kg child standing on a 5 kg skate board throws a 2 kg ball (not included in her mass) with a speed of 20 m/s in front of her. What will be her speed and direction of motion after she throws the ball?

Answer 1

The direction will be opposite to the direction in which the ball is thrown.

M₁ = mass of child = 60 kg

M₂ = mass of skateboard = 5 kg

m = mass of ball = 2 kg

v = speed at which ball is thrown = 20 m/s

u = speed of child + skateboard = ?

Apply conservation of momentum.

(M₁ + M₂)u = mv

u = mv / (M₁ + M₂)

Plug the given numbers into this last equation and get u in units of m/s.