Question

A stone is dropped into a well and is heard to hit the water 2 s after being dropped. Determine the depth of the well. Assume no time delay in the stone hitting the water and the sound being heard. $(Takeg=10m{s}^{-2})$

• A. 10 m
• B. 20 m
• C. 30 m
• D. 40 m

Answer 1

The correct option is B 20 m

• When the stone is dropped in the well, its velocity starts to increase from zero and it reaches the water in 2 seconds.
• Here, the acceleration due to gravity is uniform and in the direction of the velocity.

Therefore,
$t=2sec,a=10m{s}^{-2},u=0m{s}^{-1},s=?$
Using the second equation of motion,
$s=ut+\frac{1}{2}a{t}^2$
$s=0+0.5\times 10\times 2^2$
$s=20m$
Hence, the depth of the well is 20 m.