Balance Beam
Developed for a mobile mathart activity. See more at http://bit.ly/G3MOBILES
Finding the balance point for weights on a beam is a great intersection between math and physics.
You might start with equal weights and find the balance points, and then try for distinct masses. When the beam is released (by clicking the checkbox it will tilt or balance. You can move the fulcrum then, but not adjust where the weights are.)
This assumes that the beam is massless, so it's just a tiny bit made up. If that bothers you, put the fulcrum A at 5, and just move the masses around tto balance.