Question

A ball of mass $m$ moving with speed $u$ strikes a smooth surface at angle ${60}^o$ with the normal. If the collision is perfectly elastic, the ball will leave the surface at angle____with the normal.

• A. ${60}^o$
• B. ${53}^o$
• C. ${45}^o$
• D. ${30}^o$

Answer 1

The correct option is A ${60}^o$

Since the collision is perfectly elastic

$-\frac{v_1-v_2}{u_1-u_2}=1$

$u_2$ and $v_2$ are velocities of wall so they are equal to zero

$v_1=vsin\theta =-u_1=-usin{60}^0$

By conservation of momentum

$v=u$

$\theta ={60}^0$