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CONSERVATION OF ENERGY
The energy in a system can change as circumstances change. For example, imagine standing in a second-floor window holding a rubber ball over a sidewalk. As you hold the ball in your hand on the second floor, it has no kinetic energy because it is not moving, but it does have gravitational potential energy because it has height. When you release it and it begins to fall, the mechanical energy in the ball converts from potential energy to kinetic energy. As it loses height, the potential energy decreases; but its kinetic energy increases as it gains speed while it falls. By the time the ball reaches the ground it has no more gravitational potential energy because it no longer has height. However, it is moving its fastest just before it hits the ground, so its kinetic energy is at a maximum at that point. When it hits the ground, it briefly stops moving, so its kinetic energy goes back to zero for an instant. However, it compresses upon contact, so it gains elastic potential energy which causes it to bounce. On its way back up, this process happens in reverse, as kinetic energy decreases while it slows down and gravitational potential energy increases as it goes higher.
Energy Conversions
In the above scenario, we illustrated how energy changes from one type of mechanical energy to another. This is called an energy conversion. Energy is constantly being converted between different forms. For example, when the flow of water is used to power a hydroelectric dam, energy is converted from gravitational potential energy (the water at the top of the dam) to kinetic energy (the water flowing down through the dam & spinning turbines) to electrical energy (the spinning of magnets in a generator creates a flow of electrons) that is sent to your home and converted to radiant energy (if you turn on a light), thermal energy (if you turn on the oven), sound energy (if you turn on the radio), etc…
Law of Conservation of Energy
The law of conservation of energy tells us that energy can be converted between all of the different forms of energy but it cannot be created or destroyed. This means that the amount of energy present at the end of a process will equal the amount of energy present before it began; however, this can sometimes be difficult to see. For example, in the scenario above where you drop a ball from a second-floor window, it can appear as though energy is lost. You might expect that when the ball bounces it should reach the same height that it was dropped from if energy is conserved. After all, if it started with a certain amount of gravitational potential energy, it should have the same amount after its bounce, right? Wrong.
As the ball falls through the air, some of its energy is transferred into the air as thermal energy because of friction with the air particles. When it hits the ground, some of its energy is converted into sound energy as it causes vibrations in the air and ground. There is also friction as the rubber squeezes together, which converts more of the energy in the ball to thermal energy. Of course, as the ball travels back up after its bounce, it is again converting some of its energy into thermal energy because of friction with the air. All of these little energy conversions add up to mean that some of the gravitational potential energy the ball began with has been transferred out of the ball or converted to thermal energy, so it will not have the same amount of gravitational potential energy after the bounce. That is why a ball usually has a lower bounce height than its initial drop height. The energy is not lost or destroyed, only converted or transferred.
PUTTING THE LAW OF CONSERVATION OF ENERGY TO THE TEST