If we consider a cart of mass M ,moving along a friction-less track, with velocity v on which a boy of mass m, stands at rest initially at the front end of the cart . If the boy jumps from the cart, in the direction of motion of the cart with a velocity u relative to the cart, what will be the velocity of the boy as he emerges out of the cart. 1. Will it be simply v+u.OR 2. Should this be computed using momentum conservation as follows. Total momentum before jump = (M+m) * v Total momentum after the jump = M * vf + m * (u + vf) Where vf is the final velocity of the cart after the jump has been executed. Which is the right method.