A body of mass 1 kg is thrown upwards with a velocity $20 \mathrm{~m} / \mathrm{s}$. It momentarily

Question

A body of mass 1 kg is thrown upwards with a velocity $20 \mathrm{~m} / \mathrm{s}$. It momentarily comes to rest after attaining a height of 18 m . How much energy is lost due to air friction? $\left(g=10 \mathrm{~m} / \mathrm{s}^{2}ight)$\

Professor PC Thomas Classes · Answer

(d) : Initial velocity $u=20 \mathrm{~m} / \mathrm{s} ; m=1 \mathrm{~kg}$ Kinetic energy $=$ maximum potential energy\ Initial kinetic energy $=\frac{1}{2} 	imes 1 	imes 20^{2}=200 \mathrm{~J}$\ $\operatorname{Mgh}(\max )=200 \mathrm{~J}$\ $	herefore \quad h=20 \mathrm{~m}$\ The height travelled by the body, $h^{\prime}=18 \mathrm{~m}$\ $	herefore \quad$ Loss of energy due to air friction $$ =m g h-m g h^{\prime} $$ $\Rightarrow$ Energy lost $=200 \mathrm{~J}-1 	imes 10 	imes 18 \mathrm{~J}=20 \mathrm{~J}$\