Doppler Effect Exercises

Doppler Effect Exercises
Unless otherwise stated take the speed of sound in air to be 340 m/s.
1. An opera singer in a convertible sings a note at 600 Hz while
cruising down the highway at 90 km/hr. What is the frequency
heard by,
a) a person standing beside the road in front of the car?
b) a person on the ground behind the car?
2. A mother hawk screeches as she dives at you. You recall from
biology that female hawks screech at 800Hz, but you hear the
screech at 900 Hz. How fast is the hawk approaching?
3. 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, then you hop on your bicycle.
In which direction should you ride (toward or away from your
friend) and at what minimum speed to know if the whistle is
working?
4. A friend of yours is loudly singing a single note at 400 Hz while
racing toward you at 25.0 m/s.
a) What frequency do you hear?
b) What frequency does your friend hear if you suddenly start
singing at 400 Hz?
5. When a car is at rest, its horn emits a frequency of 600 Hz. A
person standing in the middle of the street hears the horn with a
frequency of 580 Hz. Should the person jump out of the way?
Account for your answer.
6. The security alarm on a parked car goes off and produces a
frequency of 960 Hz. As you drive toward this parked car, pass it,
and drive away, you observe the frequency to change by 95 Hz. At
what speed are you driving?
7. Suppose you are stopped for a traffic light, and an ambulance
approaches you from behind with a speed of 18 m/s. The siren on

the ambulance produces sound with a frequency of 955 Hz. What
is the wavelength of the sound reaching your ears?
8. A speeder looks in his rear-view mirror. He notices that a police
car has pulled up behind him and is matching his speed of 38 m/s.
The siren on the police car has a frequency of 860 Hz when the
police car and the listener are stationary. What frequency does the
speeder hear when the siren is turned on in the moving police car?
9. Two train whistles, A and B, each have a frequency of 444 Hz. A is
stationary and B is moving toward the right (away from A) at a
speed of 35.0 m/s. A listener is between the two whistles and is
moving toward the right with a speed of 15.0 m/s.
a) What is the frequency from A as heard by the listener?
b) What is the frequency from B as heard by the listener?
10. The siren of a fire engine that is driving northward at 30.0 m/s
emits a sound of frequency 2000 Hz. A truck in front of this fire
engine is moving northward at 20.0 m/s.
a) What is the frequency of the siren’s sound that the fire
engine’s driver hears reflected from the back of the truck?