Posted on 29 November 2011 at 15:38h

I have been doing about thermal physics with my first year classes. I always like to start with showing how thermal energy is connected to energy in mechanics. Push a block along the ground at a constant velocity, work is done but where is all the energy going etc.

So the energy is going to increase the internal energy of the ground and block but how is the energy distributed? This is what one of my students asked me after class today. It must be something to do with the atomic structure in the two surfaces but that's as far as I got so I tried to make a simple model of the situation by thinking about the collision between two pairs of blocks connected by springs.

Let's say the blocks are identical but the springs are different. The spring on the right has a k value 10 time the one on the left. When they collide the inner blocks experience an equal and opposite force (conveniently ignoring the outer blocks here). The lighter spring (left) will therefore be compressed 10 times more than the stiff one. Elastic PE = ½kx² so the left combination takes away more energy from the collision than the right hand one (k is 1/10 but x² is x100).  If we were take this to the extreme and have one spring was totally stiff then it wouldn't take any of the energy.