Q. A steel ball of radius $2\ cm$ is at rest on a frictionless surface. Another ball of radius $4\ cm$ moving at a velocity of $81\ cm/sec$ collides elastically with first ball. After collision the smaller ball moves with speed of
A) $81\ cm/sec$
B) $63\ cm/sec$
C) $144\ cm/sec$
D) None of these
Explanation
(c) Radio in radius of steel balls $= 1/2$
So, ratio in the masses $= \frac{1}{8}$ [As $M \propto V \propto r^{3}$]
Let $m_{1} = 8m$ and $m_{2} = m$
$v_{2} = \frac{2m_{1}u_{1}}{m_{1} + m_{2}} = \frac{2 \times 8m \times 81}{8m + m} = 144\ cm/s$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.