At high altitude, a body explodes at rest into two equal fragments with one fragment receiving horizontal velocity of $10 \,m/s$. Time taken by the two radius vectors connecting point of explosion to fragments to make $90^o $ is ............ $\mathrm{s}$
$10$
$4$
$2$
$1$
A ball falling freely from a height of $4.9\,m,$ hits a horizontal surface. If $e = \frac {3}{4}$ , then the ball will hit the surface, second time after .............. $\mathrm{s}$
A body dropped from a height $1\,m$ on a floor rises to a height $25\,cm$ after first rebound. The coefficient of restitution is :-
A ball hits a vertical wall horizontally at $10m/s$ bounces back at $10 m/s$
A ball moving with velocity $2 \,m/s$ collides head on with another stationary ball of double the mass. If the coefficient of restitution is $0.5,$ then their velocities after collision will be
Two particles having position vectors $\overrightarrow {{r_1}} = (3\hat i + 5\hat j)$ metres and $\overrightarrow {{r_2}} = ( - 5\hat i - 3\hat j)$ metres are moving with velocities ${\overrightarrow v _1} = (4\hat i + 3\hat j)\,m/s$ and ${\overrightarrow v _2} = (\alpha \,\hat i + 7\hat j)$ $m/s.$ If they collide after $2$ seconds, the value of $'\alpha '$ is