Benjamin Joffe

Nice work. Add surface friction too, would be cool if they slowed down :-)

They seem to slow down and speed up, how do you prevent them from all coming to a halt?

The system has zero energy loss because friction is not considered. The reason it appears to speed up and slow down is because if the large discs get some momentum then they will contain most of the collective energy but be moving slowly. If the tiny balls get knocked then they can go very fast without taking much energy from the system.

At any one point the sum of each ball's mass*velocity is constant.

You can create simple game from this neat demo probably in just 2-3 hours.
All you need is:
Add some kind of "cursor", that can be moved by mouse
Add timer
Every 3 minutes push new ball to an play arena

Player's mission is run away from balls as long as he can.

Profit!

It's a pity that you can't create global top scores board with Opera Widgets...