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##

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` 10^{8} m/s)

This can lead to enormous amounts of energy; for example, a 1 kg book
would have an energy equivalent of
9 `x` 10^{16} 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.

** Next:** Conservation of momentum
**Up:** Other types of energy
** Previous:** Wave energy
*modtech@theory.uwinnipeg.ca *

1999-09-29