4.10: First Integral- Energy Conservation and the Hamiltonian

Last updated
Save as PDF

Page ID: 29552

\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

Since Lagrange’s equations are precisely a calculus of variations result, it follows from our earlier discussion that if the Lagrangian has no explicit time dependence then:

\begin{equation}
\sum_{i} \dot{q}_{i} \frac{\partial L}{\partial \dot{q}_{i}}-L=\mathrm{constant}
\end{equation}

(This is just the first integral \(\begin{equation}
y^{\prime} \partial f / \partial y^{\prime}-f=\text { constant }
\end{equation}\) discussed earlier, now with n variables.)

This constant of motion is called the energy of the system, and denoted by E. We say the energy is conserved, even in the presence of external potentials—provided those potentials are time-independent.

(We’ll just mention that the function on the left-hand side, \(\begin{equation}
\sum_{i} \dot{q}_{i} \partial L / \partial \dot{q}_{i}-L
\end{equation}\)

is the Hamiltonian. We don’t discuss it further at this point because, as we’ll find out, it is more naturally treated in other variables.)

We’ll now look at a couple of simple examples of the Lagrangian approach.