Einstein's Mass-Energy Equation

Theorem
The energy imparted to a body to cause that body to move causes the body to increase in mass by a value $M$ as given by the equation:
 * $E = M c^2$

where $c$ is the speed of light.

Proof
From Einstein's Law of Motion, we have:


 * $\mathbf F = \dfrac {m_0 \mathbf a} {\left({1 - \dfrac {v^2} {c^2}}\right)^{\tfrac 3 2}}$

where:
 * $\mathbf F$ is the force on the body
 * $\mathbf a$ is the acceleration induced on the body
 * $v$ is the magnitude of the velocity of the body
 * $c$ is the speed of light
 * $m_0$ is the rest mass of the body.

Assume WLOG that the body is starting from rest at the origin of a cartesian coordinate plane.

Assume the force $\mathbf F$ on the body is in the positive direction along the x-axis.

To simplify the work, we consider the acceleration as a scalar quantity and write it $a$.

Thus, from the Chain Rule:
 * $a = \dfrac{\mathrm d v}{\mathrm d t} = \dfrac{\mathrm d v}{\mathrm d x} \dfrac {\mathrm d x}{\mathrm d t} = v \dfrac {\mathrm d v} {\mathrm d x}$

Then from the definition of energy:
 * $\displaystyle E = \int_0^x F \mathrm d x$

which leads us to: