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Q. A rock of mass $ m $ is dropped to the ground from a height $ h $ . A second rock with mass $ 2\, m $ is dropped from the same height. When second rock strikes the ground, what is its kinetic energy ?

UPSEEUPSEE 2009Work, Energy and Power

Solution:

Kinetic energy $\left(\frac{1}{2} m v^{2}\right)$ will be maximum when rock reaches the ground.
A heavy and light body when released from the same height reach the ground simultaneously and with same velocity, $v=\sqrt{2 g h}$
$\therefore KE \propto m$
Therefore, kinetic energy will be twice that of the first rock.