How Does the Doppler Effect Explain Frequency Changes in Moving Cars?

• Elias Waranoi
In summary, the conversation discusses the use of the Doppler formula to determine the speed and direction of a moving car based on the frequency of a sound emitted by a stationary police car. The formula is applied twice, first to find the frequency observed by the moving car and then by the police car, resulting in the correct answer of 7.02 m/s for the car's speed.
Elias Waranoi

Homework Statement

A stationary police car emits a sound of frequency 1200 Hz that bounces off a car on the highway and returns with a frequency of 1250 Hz. The police car is right next to the highway, so the moving car is traveling directly toward or away from it. (a) How fast was the moving car going? Was it moving toward or away from the police car?

Homework Equations

ƒL = ƒS(v - vL)/(v - vS)

The Attempt at a Solution

The returning frequence is larger so its wave length has to be smaller which tells us that the car is moving towards the police car.

vS = v - vƒSL = 13.76 m/s
The correct answer is 7.02 m/s.
What am I doing wrong?

Hint: You need to apply the Doppler formula twice.

1) What frequency does the moving car "observe"? (That's the frequency it will reflect.)
2) What frequency does the police car observe?

Elias Waranoi
Elias Waranoi said:

Homework Equations

Where fL is what, exactly? Is that the frequency that the police will hear?

Edit: pipped by The Doc.

Doc Al said:
Hint: You need to apply the Doppler formula twice.

1) What frequency does the moving car "observe"? (That's the frequency it will reflect.)
2) What frequency does the police car observe?

Thanks! I got it right now.

1. What is the Doppler effect?

The Doppler effect is a phenomenon in which the perceived frequency of a wave changes when the source of the wave is in motion relative to the observer. This effect is commonly experienced with sound waves, such as the change in pitch of a siren as an ambulance passes by.

2. How does the Doppler effect apply to moving sources?

The Doppler effect applies to moving sources when the source is emitting a wave, such as sound or light, and is moving towards or away from the observer. This causes a shift in the perceived frequency of the wave, resulting in a change in pitch or color.

3. How is the Doppler effect calculated for a moving source?

The formula for calculating the Doppler effect for a moving source is: f' = f(v +/- vr)/(v +/- vs), where f is the original frequency, f' is the perceived frequency, v is the speed of the wave, vr is the velocity of the receiver (observer), and vs is the velocity of the source.

4. What are some practical applications of the Doppler effect with moving sources?

The Doppler effect has numerous practical applications, including its use in radar and sonar technology, where it can be used to detect the speed and direction of moving objects. It is also used in medical imaging, such as ultrasound, to measure blood flow and detect abnormalities in the body.

5. How does the Doppler effect with moving sources differ from the Doppler effect with stationary sources?

The main difference between the Doppler effect with moving sources and stationary sources is that with moving sources, both the source and the observer are in motion, whereas with stationary sources, either the source or the observer is stationary. This results in a more complex formula and a larger range of possible frequency shifts in the case of moving sources.

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