Question

A whistle you use to call your hunting dog has a frequency of 21 kHz, but your dog is ignoring it. You suspect the whistle may not be working, but you can't hear sounds above 20 kHz. To test it, you ask a friend to blow the whistle and hop on his moped. In which direction should he ride (toward or away from you) to know if the whistle is working? Explain in words or with a diagram

Answer 1

Answer:

Away from you

Explanation:

The whistle claims to be making a frequency of 21 kHz.

We need to decrease its frequency to make it audible.

Doppler effect explains the influence of motion of the source on the observed frequency. If the source is approaching the observed frequency will be higher than the real frequency. If the source is receding the observed frequency will be lower than the real frequency

For an approaching source

$f_{observed}=f_{source}(\frac{v}{v+v_{source}} )$

For a receding source

$f_{observed}=f_{source}(\frac{v}{v-v_{source}} )$

From the above fact we understand that the friend should ride away from us to decrease the frequency and make the whistle audible.

Answer 2

The whistle you use to call has a frequency of 21 kHz, so you do not recognize it also because your threshold of hearing is only 20 kHz. Since you are unable to hear sounds above this value, you request your friend to blow the whistle and move farther away. The frequency will decrease due to Doppler effect.