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<p>Frenet vector moves about an "origin" which is the centre of the rigid object (pick some point within the object and call it its centre).  The areal velocity of the <a href="page.php?w=tangent_vector">tangent vector</a> is:<br/>
:</p>

<p>
::</p>

<p>Likewise,<br/>
:</p>

<p>
:</p>

<p>Now apply the Frenet-Serret theorem to find the areal velocity components:<br/>
:</p>

<p>
:</p>

<p>
:so that </p>

<p>
:as claimed.</p>

<p>The Darboux vector provides a concise way of interpreting <a href="page.php?w=curvature">curvature</a> ? and <a href="page.php?w=Torsion_of_a_curve">torsion</a></p><p>
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