4 A uniform lamina of mass \(m\) is in the shape of a sector of a circle of radius \(a\) and angle \(\frac { 1 } { 3 } \pi\). It can rotate freely in a vertical plane about a horizontal axis perpendicular to the lamina through its vertex O .
- Show by integration that the moment of inertia of the lamina about the axis is \(\frac { 1 } { 2 } m a ^ { 2 }\).
- State the distance of the centre of mass of the lamina from the axis.
The lamina is released from rest when one of the straight edges is horizontal as shown in Fig. 4.1. After time \(t\), the line of symmetry of the lamina makes an angle \(\theta\) with the downward vertical.
\begin{figure}[h]
\includegraphics[alt={},max width=\textwidth]{bc637a95-b469-493b-8fd4-d3b12049878b-3_257_441_1475_322}
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\caption{Fig. 4.1}
\end{figure}
\begin{figure}[h]
\includegraphics[alt={},max width=\textwidth]{bc637a95-b469-493b-8fd4-d3b12049878b-3_380_732_1635_1014}
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\caption{Fig. 4.2}
\end{figure} - Show that \(\dot { \theta } ^ { 2 } = \frac { 4 g } { \pi a } ( 2 \cos \theta + 1 )\).
- Find the greatest speed attained by any point on the lamina.
- Find an expression for \(\ddot { \theta }\) in terms of \(\theta , a\) and \(g\).
The lamina strikes a fixed peg at A where \(\mathrm { AO } = \frac { 3 } { 4 } a\) and is horizontal, as shown in Fig. 4.2. The collision reverses the direction of motion of the lamina and halves its angular speed.
- Find the magnitude of the impulse that the peg gives to the lamina.
- Determine the maximum value of \(\theta\) in the subsequent motion.