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A boy is standing on top of a tower of height $85 \mathrm{~m}$ 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 $g$ $=10 \mathrm{~m} / \mathrm{s}^{2}$, speed of sound in air $=340 \mathrm{~m} / \mathrm{s}$ )
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Verified Answer
The correct answer is:
$\mathrm 8 \mathrm{~m} / \mathrm{s}$
Time taken by sound $=\frac{85}{340}=0.25 \mathrm{sec}$
Time taken by the ball $=5.25-0.25=5 \mathrm{sec}$.
Now
$$
\begin{array}{l}
-85=u \times 5-\frac{1}{2} \times 10 \times 25 \\
\mathrm{U}=8 \mathrm{~m} / \mathrm{sec}
\end{array}
$$
Time taken by the ball $=5.25-0.25=5 \mathrm{sec}$.
Now
$$
\begin{array}{l}
-85=u \times 5-\frac{1}{2} \times 10 \times 25 \\
\mathrm{U}=8 \mathrm{~m} / \mathrm{sec}
\end{array}
$$
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