or $T=2 \pi \sqrt{\frac{3 r}{2 m}}$
or $2 \pi \sqrt{\frac{l}{\mathrm{g}}}=2 \pi \sqrt{\frac{3 \mathrm{r}}{2 \mathrm{g}}}$
$\therefore \quad l=\frac{3}{2} \mathrm{r}=\frac{3}{2} \times 1=1.5 \mathrm{m}$

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 $g =10 \ m / s ^2$ )