Question

A $60kg$ boy lying on a surface of negligible friction throws a stone of mass $1kg$ horizontally with a speed of $12m/s$ away from him. With what kinetic energy does he move back due to result of this action?

• A. $2.4$ $J$
• B. $7.2$ $J$
• C. $1.2$ $J$
• D. $36$ $J$

Answer 1

The correct option is C $1.2$ $J$

The given surface is frictionless so no external force is acting on the system. Hence linear momentum of the system will remain conserved.
$\Rightarrow$ Initial momentum $=$ Final momentum
$\Rightarrow 0=60\times V+1\times 12$
$\Rightarrow v=-\frac{1}{5}m/s$ (-ve sign show man moves in opposite direction of stone)
Kinetic energy of the boy after separation $(K.E.)=\frac{1}{2}m{v}^2$
$K.E.=\frac{1}{2}\times 60\times \frac{1}{25}$
$K.E.=1.2J$