These energies tend to be much smaller than the mass of the object multiplied by c 2, which is on the order of 10 17 joules for a mass of one kilogram. In Newtonian mechanics, a motionless body has no kinetic energy, and it may or may not have other amounts of internal stored energy, like chemical energy or thermal energy, in addition to any potential energy it may have from its position in a field of force. In the rest frame of an object, where by definition it is motionless and so has no momentum, the mass and energy are equivalent and they differ only by a constant, the speed of light squared ( c 2). Mass–energy equivalence states that all objects having mass, or massive objects, have a corresponding intrinsic energy, even when they are stationary.
Massless particles such as photons have zero invariant mass, but massless free particles have both momentum and energy. It is a fundamental physical property that is independent of momentum, even at extreme speeds approaching the speed of light (i.e., its value is the same in all inertial frames of reference). Rest mass, also called invariant mass, is the mass that is measured when the system is at rest. Because the speed of light is a large number in everyday units (approximately 3 ×10 8 meters per second), the formula implies that a small amount of rest mass corresponds to an enormous amount of energy, which is independent of the composition of the matter. The formula defines the energy E of a particle in its rest frame as the product of mass ( m) with the speed of light squared ( c 2). The principle is described by the physicist Albert Einstein's famous formula: E = m c 2. In physics, mass–energy equivalence is the relationship between mass and energy in a system's rest frame, where the two values differ only by a constant and the units of measurement.