Mass energy

Einstein in 1905 demonstrated that mass itself can be considered as a form of energy; this is the content of his famous equation

In this equation, the symbols stand for

• E: energy equivalent of mass (in Joules),
• m: mass of object (in kg),
• c: speed of light (= 3 x 10⁸ m/s)

This can lead to enormous amounts of energy; for example, a 1 kg book would have an energy equivalent of 9 x 10¹⁶ J, which is enough energy to supply the electricity needs of an average city of 750,000 people for well over one year. Compare this to the kinetic energy of the book if it is thrown at 4 m/s, which would be 8 J. Why don't we normally experience this mass energy? The answer lies in the fact that mass energy is somewhat like potential energy, in that it is only apparent or useful to us if it is converted into another form of energy, like kinetic energy. For example, a car perched on the roof of a building has a large gravitational potential energy, but that energy is not useful to us until it is converted into, for example, kinetic energy by letting the car drop to the ground and seeing the result. Similarly, in normal circumstances, mass energy remains as mass energy, and as such we do not see directly its effects. One does do so, however, in the operation of nuclear power plants and the explosion of nuclear bombs, where some mass energy is converted into other forms of energy. We shall see this in a later chapter.