A 40 kg skateboarder runs at a constant velocity of 12 m/s and jumps off a stationary skateboard with a mass of 8 kg. What is the velocity of the skateboard after the jump?

60 m/s (in the opposite direction)

Explanation:

By conservation of momentum,

Initial momentum of board + Initial momentum of man = Final momentum of board + Final momentum of man

=> 0 + 40*12 = 8 * (-v) + 0

=> v = - 60 [ Negative indicates opposite direction ]