Question

Two identical balls collide elastically with each other. Ball $B$ was in rest and ball A had speed $u$ at an angle of ${60}^{\circ }$ with line joining the centre of two balls before collision. The speed of ball $B$ after collision is $u/K$. Find $K$.

Answer 1

Before collision:

After collision:

We can resolve the momentum in a direction along the line of impact and perpendicular to it.
Velocity perpendicular to the line of impact will not change. Balls are identical and collision is elastic, so velocities get interchanged along the line of imapct.
$\therefore v_1=0,v_2=\frac{u}{2}=\frac{u}{K}\Rightarrow K=2$