1. 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 their moped. In which direction should they ride (toward or away from you) to know if the whistle is working? Explain in words or with a diagram.

Question

2.The railroad crossing lights turn red, so McKayla and her sister must stop and wait for the train to pass by. As they wait, McKayla's sister Kylie grabs her phone and uses an app to measure the frequency of the approaching train's horn. The app reads 429Hz. Assuming the train's original horn frequency is 400Hz and the speed of sound is 330m/s, how fast is the train going in m/s and miles per hour?

3. Going back to the dog whistle in question 1, what is the minimum riding speed needed to be able to hear the whistle? Remember, you can assume the following things: The whistle you use to call your hunting dog has a frequency of 21.0 kHz, but your dog is ignoring it. You suspect the whistle may not be working, but you can't hear sounds above 20.0 kHz. The speed of sound is 330m/s at the current air temperature.

Answer 1

1.
The apparent frequecy should be 20kHz i.e.lower than the actual freq. of 21kHz. Therefore the sound source should move away from you.
If the rider speed is U -
F' = v*F/(v+U)
20*10^3= 330*21*10^3/(330+U)
330+U= 330*21/20
U = 330*[21/20 -1]= 330*1/20
= 16.5m/s (min.riding speed)
2.
Train is approaching the observer.
429=330*400/[330-U]
330-U=330*400/429
U = 330[1-400/429]=22.3m/s (train's speed)