Question

A block of mass 2 kg is kept inside a lift. Lift starts moving downward with an acceleration of a = $12m/s^2$. Height of the lift is 4m. Time after which block will strike roof of the lift is (Take g = $10m/s^2$)

• A. 2 second
• B. 3.5 second
• C. 4 second
• D. 5.2 second

Answer 1

The correct option is A

2 second

Block will loose the contact with the lift and starts moving in upward direction w.r.t. the lift.

Apply 1D motion equation for the relative motion, $s_{rel}=u_{rel}t+\frac{1}{2}{a}_{rel}{t}^2\Rightarrow 4=0+\frac{1}{2}(-10-(-12\left)\right)t^2\Rightarrow 4=\frac{1}{2}\left(2\right)t^2\Rightarrow t=2s$