Sodaplay is an innovative java demonstration of simple physics.
I set up the following situation:
The friction is low or zero so that the string of masses (closed circles) bounces around. There are three fixed masses (open circles). One fixed mass is on a post in the middle and another is high up in the right upper corner. (Actually the third one at the bottom doesn't matter.) By setting the gravity (G) high the string of point masses will fall down as a chain. The first segment has a muscle (small dot), but this is overcome by the gravity.
In this image I have turned gravity off. The muscle pulls and pushes, making random pulses into the string. The net result is that the string "contracts". This is the most likely situation for the string. Even if you grab one of the free masses and pull it around the string will contract again when you let go. Indeed, this is the mechanism that rubber bands use! If you heat a rubber band it will contract because the molecules go into a higher entropy state (more disorganized) but the constraints force them to pull on each other, just as in this model. (People have even built little motors based on this principle, using sunlight to heat the rubber bands.)
origin: 2000 Apr 20
updated: 2010 Mar 04