Algodoo

used scripting but awesome.

Ok how I made it work: I put 2 balls, 1 red one and one blue one on an inner gear (could've used a circle but whatever) when the red ball hits the bottom box the density of the ball becomes 0.2 and therefor, rises up to the circle, when the now blue object that was once red hits the circle, the density of the object becomes 100, making the object fall, and the process repeats, speeding up, faster and faster.