Starting from rest, your friend dives from a high cliff into a deep lake below,

Question

Starting from rest, your friend dives from a high cliff into a deep lake below, yelling in excitement at the thrill of free-fall on her way down. You watch her, as you stand on the lake shore, and at a certain instant your keen hearing recognizes that the usual frequency of her yell, which is 903 Hz, is shifted by 55.9 Hz. Is this shift an increase or a decrease in the frequency?

a) Decrease

b) Increased

c) Can't tell from the informaiton provided

How long has your friend been in the air when she emits the yell whose frequency shift you hear? Take 344 m/s for the speed of sound in air and 9.80 m/s2 for the acceleration due to gravity.

___________s

Explanation / Answer

A. frequency will be increased.

B.

considering increasing frequesncy

f' = f*v/(v - vs)

903 + 55.9 = 903*344/(344 - vs)

vs = 20.05 m/sec

using kinematic equation

vf = vi + a*t

vi = 0

a = 9.8 m/sec^2

t = vf/a = 20.05/9.8 = 2.045 sec

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