A 47.4 kg student runs down the sidewalk and jumps with a horizontal speed of 42 m/s onto a stationary skateboard

Both the student and skateboard move down the sidewalk with a speed of 3.95 m/s. What's the skateboard's mass?

3 kg

Explanation:

Momentum before = momentum after

m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂

After jumping on the skateboard, the student and the skateboard have the same velocity, so v₁ = v₂.

m₁u₁ + m₂u₂ = (m₁ + m₂) v

(47.4 kg) (4.2 m/s) + m (0 m/s) = (47.4 kg + m) (3.95 m/s)

m = 3 kg