= Perforated-disc pendulum
{title2=$\omega^2=g|\sum_i m_i x_i|/I_O$}
For a plane lamina suspended at O, treat removed pieces as negative <masses> and attached pieces as positive ones. Add their <moments of inertia> about O using the <parallel axis theorem>, and add their first moments to locate the <centre of mass>. If the first moments lie along one axis, the gravitational restoring coefficient is $g|\sum_i m_i x_i|$. Dividing it by the total inertia gives the small-<oscillation> angular frequency squared. Both the restoring moment and inertia change when material is moved.
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