= Rolling constraint
{title2=$\mathbf v_P=0$}
For a <rigid body> rolling on a stationary support, the instantaneous contact point $P$ has zero <velocity>: $\mathbf v_P=\mathbf v_G+\boldsymbol\omega\times\overrightarrow{GP}=0$. For a wheel of radius $R$ moving horizontally and a counterclockwise <angle> $\phi$, this gives $\dot x+R\dot\phi=0$, hence $x+R\phi$ is constant. Maintaining this <mechanical constraint> generally requires sufficient <static friction> and a nonnegative <normal force>.
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