How wide is the lake if 7 seconds later she heard her own echo?

Question

Anna was on vacation and came across an echo lake. Wanting to know how far she had to swim to get across the lake to the other side, she yelled across “Hello!”

How wide is the lake if 7 seconds later she heard her own echo? Assume that the temperature is 20 ◦C and the speed of sound is 344 m/s.

Answer in units of m.

Answer 1

The round-trip time for the sound waves is 7 sec, so from Anna to the other shore is a travel time of 3.5s. Distance = vt = (344 m/s)(3.5 s) = 1204m.