Bouncing Balls (Energy) Demo

We have a basketball and a superball.  How high will they bounce if I drop them on the floor?  (almost as high as they were dropped from).  (bounce balls at a few different heights).  Notice when I drop one on the ground, it comes back up almost as high as where I dropped it from.  When physicists talk about moving objects, we often talk about energy.  There are different kinds of energy.  One kind is called kinetic energy.  Anything that is moving has kinetic energy, and the faster it is moving, the more kinetic energy it has.  Another kind of energy is potential energy.  We measure potential energy of an object as its height above the ground.  Why do we say a ball being held up has potential energy?  If the ball was dropped, gravity would accelerate it to give it kinetic energy.  So by letting go of the ball, I could change potential energy into kinetic energy.  So the question is:  why does it work that the ball always bounces back up almost to the height it was dropped from.  An important rule that physicists discovered by doing lots of experiments is that energy is conserved, which means that you can't create or destroy energy, but you can change it to different forms of energy.  So let's go through this step by step.  (go through the motions slowly with the ball)  When the ball is held up, it has a lot of potential energy and no kinetic energy.  As it falls, it starts losing it's potential energy and speeds up to get more kinetic energy.  When it hits the floor it has no potential energy, but lots of kinetic energy.  Another interesting thing happens when the ball hits the floor.  Remember that the ball bounces back up to a height lower than it started, so after one bounce it has less potential energy than it started with.  Because energy is conserved, some of that energy must have gone somewhere else.  Can anyone guess what happened to that energy?  (friction)  When the ball hits the ground, it gets squished, which causes friction between the different rubber molecules that make up the ball, and the friction heats the ball up.  In terms of energy, we would say because friction slows it down the ball, it has lost some kinetic energy, and because it has heated up, it has gained some heat or thermal energy.  Thermal energy is just another form of energy that has to do with temperature. The hotter something is, the more thermal energy it has.  So when we see the ball bounce back up almost to its original height, we know that it is always a little bit lower than it started because it has transformed some of that original potential energy to thermal energy.