General Results: Mass and Energy
Section Fifteen
This section is a bit difficult to understand, without some background knowledge.
In the Newtonian mechanics, a particle with mass m has a kinetic energy when moving with velocity v, that is, (1/2)mvv. However, in the relativistic physics, the energy of a particle is divided into rest energy and kinetic energy, and their sum, total energy comes to the foreground.
Energy = Rest Energy + Kinetic Energy
The rest energy is nothing but the mass at rest; thus Einstein claims that his theory unifies mass and energy.
However, in order to have a better understanding, you have to take the notion of momentum into consideration; in the Newtonian mechanics, this is the product of mass and its velocity, mv. The relativistic physics unifies momentum and energy, and these are different aspects of the same thing. This topic may be postponed until you understand the Minkowski 4-dimensional spacetime. See Principle of Extremal Aging.
Last modified, April 30, 2002. (c) Soshichi Uchii
suchii@bun.kyoto-u.ac.jp