Question

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

Answer 1

Ratio in radius of steel balls `= (1)/(2)`
So, ratio in their masses `= (1)/(8) (AsM 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`