Question

A ball falls from a height of $5 \, m$ and strikes a lift which is moving in the upward direction with a velocity of $1 \, m \, s^{- 1}$ , then the velocity with which the ball rebounds after collision will be

• A. $11 \, m \, s^{- 1}$ downwards
• B. $12 \, m \, s^{- 1}$ upwards
• C. $13 \, m \, s^{- 1}$ upwards
• D. $12 \, m \, s^{- 1}$ downwards

Answer 1

Answer: (B)

Velocity after a fall of $5 \, m$ is,
$u_{1}=\sqrt{2 \times 10 \times 5}=10 \, m \, s^{- 1}$

$\left(\bar{v}\right)_{1}-\left(\bar{v}\right)_{2}=e\left(\left(\bar{u}\right)_{2} - \left(\bar{u}\right)_{1}\right)$
$v_{1}-1=1-\left(- 10\right)$
$v_{1}=12 \, m \, s^{- 1}$