Question

As friends frequently do, Josh and John stand at either end of a very long, stationary skateboard, with Josh at the front and John at the rear. John then tosses a heavy ball toward Josh who, caught unawares, ducks so that the ball goes sailing over his head and lands on the ground. There is no friction between the skateboard and the ground. After the ball is thrown, which way, if at all, does the skateboard move
A.Backward
B.Forward
C.Neither

Answer 1

Answer:

B. Forward

Explanation:

The net force in the system is initially balanced and the system is in equilibrium.

1) As John ducks, two things happens. First, there is a sudden impulse from his end which will change the momentum of the system from zero to an amount Mv proportional to the impulse Ft, which will be directed away from his direction because,

2) the process of ducking pushes his center of mass forward, which also moves the center of mass of the whole system away from him. This two effect will cause the skateboard to move forward.