Question

A person in lift which ascends up with acceleration $10m/s^2$ drops a stone from a height $10m$. The time of descent is $(g=10m/s^2)$

• A. $1s$
• B. $2s$
• C. $1.5s$
• D. $0.5s$

Answer 1

The correct option is A $1s$

Since, the lift is ascending up against the force of gravity with a = $10m/s^2$, we have
Apparent weight of the man $M(g+a)$ or the acceleration is $20m/s^2$(also called pseudo acceleration).
Now, the stone is dropped from height $h=10m$ and hence time of descent $t=\sqrt{\frac{2h}{g+a}}$
$t=\sqrt{\frac{2\times 10}{20}}=1sec$