Choose an example in which the momentum of a system is not constant. Choose an example in which the momentum of a system is not constant. A freely falling metal ball, with the ball and Earth as the system. A frog on a floating piece of wood jumping off the wood, with the frog and the wood as the system. A freely falling metal ball, with the ball as the system. It is not possible to give an example since the momentum of a system is always constant.