
spring is stretched by a distance $\frac{\mathrm{L}}{2} \theta .$ Therefore
restoring force is spring $=\mathrm{K}\left(\frac{\mathrm{L}}{2} \theta\right) .$ Each spring
causes a torque $\tau(=\mathrm{Fd})=\left(\frac{\mathrm{KL} \theta}{2}\right) \frac{\mathrm{L}}{2}$ in the
same direction.
Therefore equation of ratational motion is $\tau=I \alpha$
$-2\left(\frac{\mathrm{KL} \theta}{2}\right) \frac{\mathrm{L}}{2}=\mathrm{I} \alpha$ with $\mathrm{I}=\frac{\mathrm{ML}^{2}}{12}$
$\alpha=-\left(\frac{6 K}{M}\right) \theta$
Standard equation of angular $S.H.M.$ is
$\alpha=-\omega^{2} \theta$
$\omega^{2}=\frac{6 \mathrm{K}}{\mathrm{M}}$
frequency $\quad \mathrm{f}=\frac{\omega}{2 \pi}=\frac{1}{2 \pi} \sqrt{\frac{6 \mathrm{K}}{\mathrm{M}}}$
Simultaneously at $t=0$, a small pebble is projected with speed $v$ from point $P$ at an angle of $45^{\circ}$ as shown in the figure. Point $P$ is at a horizontal distance of $10 \ cm$ from $O$. If the pebble hits the block at $t=1 \ s$, the value of $v$ is (take $g =10 \ m / s ^2$ )

(where $g =$ acceleration due to gravity)