Question

A stone is dropped from the top of a $400\text{m}$ high tower. At the same time another stone is projected vertically upwards from the ground with a speed of $50\text{m/s}$. The two stones will cross each other after a time

• A. $2\text{s}$
• B. $4\text{s}$
• C. $6\text{s}$
• D. $8\text{s}$

Answer 1

The correct option is D $8\text{s}$


$S_{21}=u_{21}t+\frac{1}{2}{a}_{21}{t}^2$
But $a_{21}=-g-(-g)=0{\text{ms}}^{-2}$
$\Rightarrow +400=[50-0]t\Rightarrow t=8\text{s}$