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//see` collides elast cally with first ball. After collision the smaller ball moves with speed of

Options

A. 81 cm/sec
B. 63 cm/sec
C. 144 cm/sec
D. None of these

Answer

C

Explanation

Ratio in radius of steel balls `=1//2`
So, ratio in their masses `=(1)/(8)` [As `M prop V prop r^(3)`]
Let m`_(1)= 8m and m_(2)=m`

`v_(2)= (2m_(1) u_(1))/(m_(1) + m_(2))= (2 xx 8m xx 81)/(8m + m)= 144 cm//s`