A boy, mass 70.0 kg, riding a skateboard, mass 2.0 kg, is traveling 3.0 m/s East when he attempts to jump forward from his skateboard. If his velocity immediately after leaving the skateboard is 3.1 m/s, what is the velocity of the skateboard?

0.5 m/s west

Explanation:

Momentum before = momentum after

m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂

(70.0 kg) (3.0 m/s) + (2.0 kg) (3.0 m/s) = (70.0 kg) (3.1 m/s) + (2.0 kg) v

210 kg m/s + 6 kg m/s = 217 kg m/s + (2.0 kg) v

-1 kg m/s = (2.0 kg) v

v = - 0.5 m/s

The skateboard's velocity is 0.5 m/s west.