# Doppler effect, moving car and whistle

• cheff3r
In summary, Sally, a police officer, blows her whistle at a frequency of 900 hz to signal a car driven by David to stop as it approaches the intersection at a speed of 75 km/h. The frequency that David hears is calculated to be 954.65 Hz. As David does not stop and accelerates to 90 km/h, Sally blows her whistle again and the frequency that David hears is calculated to be 834.40 Hz. This is due to the Doppler effect, where the frequency of a wave is affected by the relative motion between the source and the detector.
cheff3r

## Homework Statement

Sally is a police officer who is standing in an intersection. A car driven by David approaches the intersection at a speed of 75 km/h. Sally blows her whistle (at a frequency of 900 hz) and signals the driver to stop. What does frequency does David hear? (speed of sound is 343 m/s). David does not stop and passes through the intersection and accelerates to 90 km/h, sally blows her whistle again, what frequency does David hear?

## Homework Equations

f'=(v-(v(D)-v(M)))/(v-(v(S)-v(M))*f

## The Attempt at a Solution

So the velocity of the median is zero, v is speed of sound =343 m/s , v(D) is David and at 75 km/h = 20.83 m/s and v(S) is sally = 0 and i also make it negative since traveling at sally
so i go
f'=(343-(-20.83))/343*900 = 954.65 Hz

next I do the same as above except i make the v(D) = 90 km/h = 25 and is positive
hence
f'=(343-25)/343*900 = 834.40Hz this is my problem shouldn't the traveling away Doppler affect give a greater frequency than started with?

Last edited:
No, traveling away should lower the frequency, and traveling towards should increase the frequency. When you move towards someone each successive wavefront is closer together than expected, when you move away from someone each successive wavefront is farther apart than expected (hence, lower frequency).

So my method is right? If anyone has spare time on their hands to double check my answers that will be much appreciated (i'm a little foggy in the air and would love to know if I'm getting the right answer)

I don't know about the figures since I'm too lazy to plug in numbers, but the concept is right. For a detector moving away from a source, the frequency should be lower than at the source, and for a detector moving toward a source the frequency should be higher.

## 1. What is the Doppler effect?

The Doppler effect is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. It is commonly observed 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 a moving car and a whistle?

If a car is moving towards an observer, the sound waves from the whistle will be compressed and have a higher frequency, resulting in a higher pitch. If the car is moving away, the sound waves will be stretched and have a lower frequency, resulting in a lower pitch.

## 3. Can the Doppler effect be observed with light?

Yes, the Doppler effect also applies to light waves. This is known as the "redshift" or "blueshift" depending on whether the light source is moving away or towards the observer, respectively.

## 4. How is the Doppler effect used in science and technology?

The Doppler effect is used in a variety of ways, including in radar and sonar systems to measure the speed and location of objects, in medical ultrasound to detect blood flow and heart rate, and in astronomy to determine the speed and direction of celestial objects.

## 5. What are the limitations of the Doppler effect?

The Doppler effect is based on the assumption that the source of the wave and the observer are moving relative to each other. It also does not account for other factors that may affect the frequency or wavelength of a wave, such as wind or temperature. Additionally, the Doppler effect is not applicable for objects that are moving at speeds close to the speed of light.

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