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Potential Energy
       Energy comes in two forms: potential and kinetic. Potential energy is energy that is stored. Kinetic energy is the energy of motion. It is energy that is being applied, such as in moving an object, moving heat or light, or moving an electric current. Most often, people think of kinetic energy when they hear the term “energy,” but potential energy is just as important.
       There are three forms of potential energy. They are due to position, condition or composition. Whenever something is moved to a higher level, it has gained potential energy due to position as it overcomes gravity. When a stress is introduced, an object gains potential energy due to condition. The energy present in chemical bonds is due to composition. This is the potential energy present in food and all fuels – oil, gas, coal and wood are common examples.
       It is easy to see examples of potential energy. Raise a ball and it now has potential energy. That means it has stored energy that can be transformed into kinetic energy. If you stretch a rubber band, it now has potential energy. Similarly, if you set a mouse trap, it has potential energy in the spring. Many simple toys operate by storing potential energy that can then be released.
       Potential energy can be transformed into kinetic energy and kinetic energy can be transformed into potential energy. Potential energy in the form of composition is stored in your cells and muscles in compounds such as adenosine triphosphate (ATP). This energy can be changed into kinetic energy that can then be changed once again into another form of potential energy by raising a ball to a higher level. If the ball is released, it starts to fall, once again changing to kinetic energy. When the ball hits the floor, it is compressed, again changing to potential energy. This is them once more changed to kinetic energy as the ball starts to move upward.
       When Doc Edgerton invented high speed photography, it made it possible to see the potential energy stage when a football, tennis ball or golf ball is compressed in the momentary interim stage between having the ball moving in one direction (kinetic energy) to then moving in another direction (again, kinetic energy). In between, it existed as potential energy. There is always a transfer from one form to another. When a ball is dropped, some energy is “lost” when it hits the floor, both as sound and as heat, which is why the ball does not rise to the original level. To use a yoyo, a slight upward pull is necessary when the yoyo is at its lowest point to replace the energy that was lost. Otherwise, it won’t return all the way to your hand.
       An interesting demonstration is dropping a ball and getting it to go higher than it was when it was dropped, which seems to disagree with the law of thermodynamics that requires some energy to always be lost in these exchanges. The way to do this is to simultaneously drop a pair of balls, one sitting atop the other. As they hit the ground, the lower ball winds up transferring much of its energy to the upper ball. The lower ball may only bounce up a short distance while the upper ball can shoot skyward. The net result is less than the original potential energy when the balls were dropped, but the upper ball now has much more kinetic energy, which takes it higher as that energy is again being converted to potential energy.
      
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