Question

A lift with open top is moving up with acceleration $f$. A body is thrown up with speed $u$relative to lift and it comes back to lift in time. Then speed $u$is (g=acceleration due to gravity)

• A. $ft+gt$
• B. $\sqrt{{\left(ft\right)}^2+{\left(gt\right)}^2}$
• C. $\frac{ft+gt}{2}$
• D. $\frac{f^{2}t}{g}$

Answer 1

The correct option is C $\frac{ft+gt}{2}$

In a lift,

Net acceleration= $a+g=f+g$

Time of flight= $t=\frac{2u}{f+g}$

$\Rightarrow ft+gt=2u$

So, $u=\frac{ft}{2}+\frac{gt}{2}$