Question

A boy is standing on top of a tower of height $85m$ and throws a ball in the vertically upward direction with a certain speed. If $5.25$ seconds later he hears the ball hitting the ground, then the speed with which the boy threw the ball is (take $g=10m/s^2$, the speed of sound in air $=340m/s)$:

• A. $6m/s$
• B. $8m/s$
• C. $10m/s$
• D. $12m/s$

Answer 1

The correct option is B $8m/s$

Time taken by sound $=\frac{85}{340}=0.25sec$
Time taken by the ball $=5.25-0.25=4sec$.
Now
$-85=u\times 5-\frac{1}{2}\times 10\times 25$
$u=8m/sec$.