A bat locates insects by emitting ultrasonic “chirps” and then listening for echoes from the bugs. Suppose a bat chirp has a frequency of 25 kHz. How fast would the bat have to fly, and in what direction, for you to just barely be able to hear the chirp at 20 kHz?
The bat should fly at the speed of away from you.
The frequency of bat chirps is
A bat must fly away from you so that the chirp frequency observed by you is less than 25 kHz, that is, .
Speed of sound in air is .
Doppler effect for receding source is given by.
Substitute the given values
Therefore, the bat should fly at the speed of away from you.
Oil explorers set off explosives to make loud sounds, then listen for the echoes from underground oil deposits. Geologists suspect that there is oil under 500-m-deep Lake Physics. It’s known that Lake Physics is carved out of a granite basin. Explorers detect a weak echo 0.94 s after exploding dynamite at the lake surface. If it’s really oil, how deep will they have to drill into the granite to reach it?
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