A stone is dropped from the top of a tower 400 ft high into a lake. How many seconds will it take before the impact sound is heard at the top of the tower?

5 seconds

4.5 seconds

3.5 seconds

6 seconds

What happens: Stone falls, hits water, sound travels back up.
Key points:
- Time to fall = $sqrt{frac{2h}{g}} = sqrt{frac{2 times 400}{32}} = 5 text{ seconds}$
- Time for sound to travel up = $frac{400}{1120} approx 0.36 text{ seconds}$
Total time: Around $5 + 0.36 = 5.36$ seconds (approximate).