Q. 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
Explanation
(a) Kinetic energy$\left( \frac{1}{2}mv^{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.
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.