The kinetic energy of an object is 72 J. If the mass of the object is 4 kg then what must be the velocity of the body?

Given:
Mass, m= 4 kg
Kinetic energy, KE = 72 J

To find:
Velocity, v

Solution:
The kinetic energy of a particle is given by $\mathrm{K}.\mathrm{E}=\frac{1}{2}\times {\mathrm{mv}}^2$
$\Rightarrow 72=\frac{1}{2}\times 4\times {\mathrm{v}}^2$
$\Rightarrow {\mathrm{v}}^2=36$
$\Rightarrow \mathrm{v}=6 \mathrm{m}/\mathrm{s}$

Therefore, the velocity of the object must be 6 m/s.