Question

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

• A. Twice that of the first rock
• B. Four times that of the first rock
• C. The same as that of the first rock
• D. Half that of the first rock

Answer 1

Answer: (A)

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, $v=\sqrt{2gh}$
$\therefore KE\propto m$
Therefore, kinetic energy will be twice that of the first rock.