 When an object falls and hits the earth, it may bounce. The height to which it rebounds will allow you to calculate the elasticity of its collision with the earth. Some collisions are completely inelastic ... like dropping a big glob of mud. For a basketball, the rebound height may be as much as 80%; some rubber compounds will allow a return to over 90% of the original height. But a perfectly elastic collision is impossible. In other words, if the ball is just dropped, the ball will never return to its original height. Eventually it will stop bouncing altogether. Why is this? When the ball first falls off the edge of the platform, it has a potential energy based on its height above the ground. After falling, at the instant before it hits, all that potential energy has been turned into kinetic energy. As the ball meets the ground, it deforms, and some of the kinetic energy gets stored in the molecules of the ball as they bend, just like in a spring. But some of the energy is lost as heat, as the molecules twist, and some more goes into warming the floor slightly. At maximum deformation, all the energy of the fall has either been stored in the ball, or dissipated as heat. Then the ball rebounds upward, releasing the potential energy as kinetic energy. As it does this, more energy is lost to heat as the molecules twist and unbend. The result is that, when the ball starts back upwards, it has less energy than when it began its fall. There is no way to stop this from happening. This is the Second Law of Thermodynamics, and can be loosely defined as 'energy always escapes'. Most of modern engineering practices are a collection of methods for making this energy loss as small as possible. But it's always there! Incidentally, this is the reason that so-called 'perpetual motion' devices are impossible. |