A soccer ball of mass 1kg is moving at a speed of 8 m/s.

    (a)   What is the magnitude of the momentum of the ball ?                                                 (4)

    (b)   What is the kinetic energy of the ball ?                                                                      (4)


    (c)    The ball stikes a brick wall and rebounds with a speed of 5 m/s. Calculate the change in
            momentum of the ball.                                                                                                (6)

Assuming 1D rebound



Direction is away from the wall.


    (d)   If the ball is in contact with the wall for 0.1 s, what is the magnitude of the force exerted
           by the ball on the wall ?                                                                                               (6)




    (e)    The wall remains at rest relative to the earth. Is the momentum of the system comprising
             the wall and the ball conserved ? If not, why not ?                                                   (5)

If wall and ball conserve momentum then

But  Δpwall = 0 and Δpball ≠ 0

Therefore momentum is not conserved for this system.  This is because Fext ≠ 0 : the wall is attached to the earth which exerts a force on it keeping it stationary.  If the earth was included in the “system” momentum would be conserved.