Question

A stone is dropped into a lake from a tower 500 metre high. The sound of the splash will be heard by the man approximately after

• A. 11.5 seconds
• B. 21 seconds
• C. 10 seconds
• D. 14 seconds

Answer 1

Answer: (A)

11.5 seconds

Answer 2

The time taken by the stone to reach the lake

$t_{1} = \sqrt{\left( \frac{2h}{g} \right)} = \sqrt{\left( \frac{2 \times 500}{10} \right)} = 10sec$ (Using $h = ut + \frac{1}{2}g ⥂ t^{2}$)

Now time taken by sound from lake to the man

$t_{2} = \frac{h}{v} = \frac{500}{340} \approx 1.5\sec$

⇒ Total time $= t_{1} + t_{2} = 10 + 1.5 = 11.5sec.$