If we throw a body upwards with velocity of 4 ~m / s , at what...

Key Idea : At a given height the half of the kinetic energy of the body is equal to its potential energy. Initial kinetic energy of the body $$\begin{aligned} & =\frac{1}{2} m v^2 \\ & =\frac{1}{2} m(4)^2=8 m\end{aligned}$$ Let at height $h$, the kinetic energy reduces to half, ie, it becomes 4 m . It is also equal to potential energy. Hence, $m g h=4 \mathrm{~m} or h=\frac{4}{g}=\frac{4}{10}=0.4 \mathrm{~m}$