Posted on 25 October 2011 at 16:03h

We were discussing an exam question in class and discussing whether momentum is conserved if a system is acted upon by external but balanced forces. To illustrate the point I tried to think of an example where the forces would be balanced but momentum conserved. The only example I came up with was a box on the floor with a ball bouncing back and forth inside it. Now the box is not moving so the forces are balanced but momentum is not conserved as the ball is going back and forth. As is always the case I realised my mistake on the way home. When the ball hits the wall of the box it exerts a force on it, to prevent the box moving there must be an equal and opposite force on the box, this is friction. When  got home made an example in interactive physics where I had two constant forces acting on the box. In this case the momentum is conserved as the box moves back and forth.

This video shows what its like if the forces are constant and balanced.

This one shows how the friction and tension change as the ball hits the sides.