A hiker determines the length of a lake by listening for the echo of her shout reflected by a cliff at the far end of the lake. She hears the echo 2.5 s after shouting. Estimate the length of the lake.
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This is Giancoli Answers with Mr. Dychko. When this hiker shouts, her sound goes from her mouth all the way to the cliff and then has to bounce off and then come back to her ears and then she'll hear the echo. And it takes 2.5 seconds for this full round trip. So, the distance of the sound travels, that is from the hiker to the cliff and back, that total distance is the speed of sound multiplied by the total time. But this distance is 2 times the width of the lake. So, d is 2 l. and so we'll put in 2 times length in place of d and that equals v t, and divide both sides by 2. And the length of the lake is v t divided by 2. So, that's 343 meters per second times 2.5 seconds divided by 2, is about 430 meters.
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