= Pendulum with a freely sliding support
{title2=$M_{\rm eff}(\phi)=m_2l^2(m_1+m_2\sin^2\phi)/(m_1+m_2)$}
For a pendulum whose support slides without horizontal <force>, the support coordinate is cyclic and total horizontal <momentum> is conserved. At zero horizontal <momentum>, eliminating support <velocity> leaves the displayed angle-dependent effective inertia. The <energy> is $M_{\rm eff}\dot\phi^2/2-m_2gl\cos\phi$. Fixing the <momentum> before evaluating motion is distinct from fixing the support position.
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