a. A rail road flat car of mass M can roll without friction along a straight horizontal track . Initially, a man of mass m is standing on the car which is moving to the right with speed v0. What is the change in velocity of the car if the man runs to the left so that his speed relative to the car is vrel just before he jumps off at the left end? b. If there are n men each of mass m on the car, should they all run and jump off together or should they run and jump one by one in order to give a greater velocity to the car?