The sound source of a ship’s sonar system operates at
a frequency of 18.0 kHz. The speed of sound in water (assumed
to be at a uniform 20°C) is 1482 m/s. What is the difference
in frequency between the directly radiated waves and the waves
reflected from a whale traveling directly toward the ship at
4.95 m/s ? The ship is at rest in the water.
fship = (v+vship/v+vwhale)fwhale
The Attempt at a Solution
To find the frequency of the reflected waves, I used the above equation with vship=0 and vwhale=-4.95m/s. Found the reflected frequency to be 18060.32Hz, which means a 60Hz difference. My textbook says it should be double that, 120Hz.