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Q.
A splash is heard after $3$ second after the stone is dropped into a well of depth $20\, m$. The velocity of sound is:
Motion in a Straight Line
Solution:
Stone is dropped, $u=0$
Let $t$ is the time taken by the stone to cover depth of $20 m$.
$20=u t+\frac{1}{2} g t^{2} \Rightarrow 20=\frac{1}{2} \times 10 \times t^{2}$
$t=2$
Splash heard after $3$ second but time taken by stone is $2$ second
$\therefore \Delta t=3-2=1$ second
or velocity of particle $=20 / 1=20\, m / s$