Sound A whale emits of sound of 50Hz to tell a wayward calf to catch up to the p

Question

Sound

A whale emits of sound of 50Hz to tell a wayward calf to catch up to the pod. The whale is moving due East at 5 m/s while the calf (1.2 Km away) is moving due West at 4 m/s. The speed of sound in water is 1500 m/s and the speed of sound in air is 340 m/s. a) How long does it take the sound to reach the calf? What is the frequency of the sound received by the calf? b) The whale now waits near the surface of the water for the calf. The whale continues to emit the 50 Hz sound, and some of the sound is transmitted into the air. What is the frequency and the wavelength of the sound in the air?

Explanation / Answer

a)We know that

speed = dist/time

time = dist/speed = 1.2 x 10^3 m/1500 m/s = 0.8 s

Hence, t = 0.8 s

Both are moving away from each other, so the frequency that calf heards would be:

f' = (vw - vc)/(vw + vc) f = (5 - 4)/(5 + 4) x 50 = 5.56 Hz

Hence, f' = 5.56 Hz

b)The frequency remains unchanged. It does not change while chaging the medium as it depends on the source only

f = 50 Hz

v = f lambda => lambda = v/f = 340/50 = 6.8 m

Hence, lambda = 6.8 m ; f = 50 Hz

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