# Conservation of Energy

## Theorem

Let $P$ be a physical system.

Let it have the action $S$:

$\displaystyle S = \int_{t_0}^{t_1} L \rd t$

where $L$ is the standard Lagrangian, and $t$ is time.

Suppose $L$ does not depend on time explicitly:

$\dfrac {\partial L} {\partial t} = 0$

Then the total energy of $P$ is conserved.

## Proof

By assumption, $S$ is invariant under the following family of transformations:

$T = t + \epsilon$
$\mathbf X = \mathbf x$
$\nabla_{\dot {\mathbf x} } L \cdot \boldsymbol \psi + \paren {L - \dot {\mathbf x} \cdot \nabla_{\dot {\mathbf x} } L} \phi = C$

where $\phi = 1$, $\boldsymbol \psi = \mathbf 0$ and $C$ is an arbitrary constant.

Then it follows that:

 $\displaystyle L - \dot {\mathbf x} \cdot \nabla_{\dot {\mathbf x} } L$ $=$ $\displaystyle T - U - \dot {\mathbf x} \cdot \nabla_{\dot{\mathbf x} } \paren {T - U}$ $\displaystyle$ $=$ $\displaystyle T - U - \dot {\mathbf x} \cdot \nabla_{\dot{\mathbf x} } \paren{\frac m 2 \dot {\mathbf x}^2}$ $\displaystyle$ $=$ $\displaystyle T - U - m \dot {\mathbf x}^2$ $\displaystyle$ $=$ $\displaystyle T - U - 2 T$ $\displaystyle$ $=$ $\displaystyle - \paren {T + U}$

Since the last term is the total energy of $P$, we conclude that it is conserved.

$\blacksquare$