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The Law of Conservation of Energy states that energy cannot be created or destroyed, just transformed from one form to another. These forms can include kinetic and potential energy as well as light, heat and sound. Energy can also be transferred between objects. Since energy cannot be created or destroyed, the amount of energy present in the universe is always the same. It is simply being transformed and transferred over and over again.

The effect of friction

Imagine a bouncing ball. When the ball is in the air, it has gravitational potential energy. As it falls, the potential energy the ball has is transformed into kinetic energy. When the ball hits the ground, it becomes deformed and gains elastic potential energy. As the ball bounces back up, it gains kinetic energy again, but as it rises, the kinetic energy is transformed into gravitational potential energy once again.

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Judging by the fact that energy cannot be created or destroyed, it seems reasonable that the ball should continue bouncing forever. Observing a bouncing ball on Earth, however, shows that eventually the ball will slow down and stop bouncing. Where does the extra energy go? Most of the energy found in a bouncing ball is transformed into heat and sound due to friction. As the energy is transformed into heat and sound, it is not available to be transformed into gravitational potential energy, elastic potential energy or kinetic energy. Some of this friction takes place with the air, while some of it takes place with the ground.

Another example is driving in a car. In this case, the petrol in the car's fuel tank provides energy to push the car forward. Once that energy has been used, however, it cannot be recovered. The energy is transformed into heat and sound through friction with the air and the ground and combustion within the engine. Some cars, such as hybrid cars, are able to transform some of the excess energy into potential energy, but the transfer is not complete. Some energy is still lost to heat and sound.

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The term efficiency can be used to describe how much energy is lost through friction. A more efficient process means that more energy is retained, while a less efficient process retains less energy.


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