0.23 Phy1210: energy -- internal and external forces  (Page 3/4)

Another trip involving only internal forces

On the way up from the bounce, the potential energy due to gravity will be increasing and the kinetic energy will be decreasing until the ball reaches thetop of the bounce.

For an instant, the kinetic energy will be zero and the gravitational potential energy will be at its maximum. Then the ball will begin its trip backtoward the floor, gaining kinetic energy and losing potential energy along the way.

Application of another external force

Then the ball will strike the floor again, and the process will repeat. Each time the ball strikes the floor, some of the total mechanical energy will belost, having been converted into some other form of energy such as thermal energy, sound wave energy, etc.

All of the potential energy is expended

Eventually, an amount of energy equal to the original potential energy will have been converted into those other forms of energy and the ball will come torest on floor. However, because of the design of the Super Ball, many cycles of the process may be required for the ball to come to rest.

An aside -- a ball of Play Doh

Consider what would happen if instead of being a Super Ball, the toy were a ball of Play Doh (a toy manufactured by Hasbro that doesn't bounce very well) . The ball of Play Doh would probably only make one trip from the tabletop to the floor.

On the way down, the ball of Play Doh would exchange potential energy for kinetic energy. When it hits the floor, all of the kinetic energy would probablybe expended by converting it to heat energy, sound energy, etc., and by deforming the ball of Play Doh beyond its elastic limit. (Play Doh has a verylow elastic limit, if any.)

Not done with the Super Ball yet

Now let's get back to the case of the Super Ball. Keep in mind that the total mechanical energy possessed by the ball when it comes to rest on the floor isnot zero, because it still feels an attraction due to the force of gravity. Thus, it still possesses gravitational potential energy.

If all of this has been happening in a second-story loft, and the cat comes along again and causes the ball to roll off the edge of the loft floor, aprocess similar to that described above will occur all over again involving the ball and the floor below the loft.

Another phenomena

There is another phenomena that would probably occur with the Super Ball, but which would be visible only through the use of high-speed photography or othersensitive technology.

Oscillations

The impact with the floor would probably cause the ball to go into spring-like oscillations. By this I mean that it would be compressed by theimpact with the floor. When it expands, (in addition to causing it to bounce upward), the ball would probably over-expand or stretch and gain elasticpotential energy in the stretched configuration.

Energy conversion

During this process, the potential energy due to compression would be converted to kinetic energy as the molecules in the ball fly away from oneanother, and then be converted into potential energy again as the molecules fly too far away from one another (stretch).