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 $g =10 \ m / s ^2$ )
$T =\frac{2 u \sin \theta}{ g }=1 sec . $
$\frac{2 u \sin 45}{g}=1 sec \text {. } $
$u=\frac{g}{\sqrt{2}} $
$u=\sqrt{50} \ m / s $



