Question

A ball after falling a distance of 5 meter from rest hits floor of a lift and rebounds. At the time of impact the lift was moving up with a velocity of $1m/sec$. The velocity with which the ball rebounds just after impact is-($g=10m/se{c}^2$)

• A. $10m/sec$
• B. $11m/sec$
• C. $12m/sec$
• D. $13m/sec$

Answer 1

The correct option is C $12m/sec$

When a lighter object having velocity $v_L$ collide with much heavier object having velocity $v_H$ both moving in opposite direction, then after collision, lighter object recoil in the opposite direction with velocity $v_L+{2v}_H$

Velocity of ball at impact $=v_L=\sqrt{2gh}=\sqrt{2\times 10\times 5}=10m/s$

Velocity of lift $=v_H=1m/s$

Recoil velocity of ball $=10+2\times 1=12m/s$