an> then:</p>
<div class="math notranslate nohighlight">
\[\begin{split}E_r =
\begin{equation*}
 \begin{cases}
   \displaystyle\frac {2 I r} {a ^ 2\upsilon}, r &lt;a,
    \\
   \displaystyle\frac {2 I} {r\upsilon}, r&gt; a.
 \end{cases}
\end{equation*}\end{split}\]</div>
</div>
<div class="section" id="Equations-of-motion">
<h2>Equations of motion<a class="headerlink" href="#Equations-of-motion" title="Permalink to this headline">¶</a></h2>
<p>Newton’s second law <span class="math notranslate 