Question

How fast should a man of 50 kg run so that his kinetic energy be 625 J?

Answer 1

We have to find the speed of the running man. We have,
Mass of the man (m) = 50 kg
Kinetic energy of the man (K.E) = 325 J
So, we use the expression of kinetic energy to find the unknown quantity as,
$K.E=\frac{1}{2}m{v}^2$
Where,
KE - Kinetic Energy
m - Mass
v - Speed
So,
$$\begin{gathered} v=\sqrt{\frac{2(K.E)}{\left(m\right)}} \\ \mathrm{Therefore}, \\ v=\sqrt{\frac{2\left(625\right)}{50}}\mathrm{m}/\mathrm{s} \\ =\sqrt{25}\mathrm{m}/\mathrm{s} \\ =\boxed{5\mathrm{m}/\mathrm{s}} \end{gathered}$$
So, the man should be running with a speed of 5 m/s.