Relativity

Why is the speed of light squared in E=MC²?
• ### Science Channel

1. It's the most famous equation ever, suggesting a symmetry between mass and energy, with the incredible revelation that you can get an enormous amount of energy out of a very small amount of mass. But what does the speed of light squared have to do with it? It all breaks down to kinetic energy, which is proportional to mass. If you accelerate an object, the kinetic energy increases to the tune of the speed squared. For instance, if you double your speed in an automobile, the braking distance is four times longer. In other words, the braking distance is equal to the speed squared. Interestingly, Einstein's use of "C" relates more to the fact that it is the speed where time and space are in some ways the same than the fact that it was the speed of "light" [source: U.S. Department of Energy Office of Science and Technology].

Kinetic energy, to take a brief primer, is the energy of an object in motion, and the more mass or velocity a moving object has, the greater its kinetic energy released. Today, many electric cars take advantage of kinetic energy to operate more efficiently, using the car's kinetic energy to charge its batteries. Kinetic energy is distinct from another kind of energy -- potential energy, stored by an object. You may think of batteries, such as those in the electric car, as stored, potential energy, but in fact, all sorts of objects contain potential energy, objects you might not even expect. For instance, by placing a ball at the top of a hill, you're effectively storing the energy that it took to move the ball up the hill in the ball itself. The higher the hill and the heavier the ball, the more energy is required to move the ball up the hill (and, accordingly, the more potential energy is contained in the ball). Even a rubber band, when pulled back as far as it can go, contains potential energy in that stretched position.

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