Sound: freq moving sources

In summary, the question discusses a train traveling at 120km/h and blowing its 400Hz whistle. The speed of sound is taken to be 340m/s and the question is what frequency will be heard. The formula used to calculate the frequency is f'=f/(1-vs/vo). There are possible answers listed, and the person asking the question realizes they made a mistake in their calculation. They are looking for help in finding the correct answer.
  • #1
Dx
A train is traveling toward you at 120km/h. The train blows its 400Hz whistle. take the speed of sound to be 340m/s. what's the frequency do you hear?

f'=f/(1-vs/vo)
400/(1-(4.32x10^8 / 340)) i got 4.32x10^8 = 120000m * 3600sec

what am i doing wrong?
The possible answers are: which is it and what have i done wrong to miss this test question?

a) 444hz
b) 364hz
c) 361 hz

thanks!
Dx :wink:
 
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  • #2
Originally posted by Dx
f'=f/(1-vs/vo)
400/(1-(4.32x10^8 / 340)) i got 4.32x10^8 = 120000m * 3600sec

You made a mistake converting the speed of the source from km/h to m/s. If you look at it, you have the source moving faster than light, which is a big no-no.
 
  • #3


Based on the given information, the correct answer is b) 364hz. To arrive at this answer, you have to use the formula for frequency of a moving source, which is f'=f/(1-vs/vo). In this case, f is the original frequency of the whistle, which is 400Hz. vs is the speed of the source, which is the train moving at 120km/h or 33.33 m/s. vo is the speed of sound, which is given as 340m/s. So, the equation becomes f' = 400/(1-33.33/340) = 364hz. Therefore, the frequency you hear is 364hz. You may have made a mistake in your calculations, which is why you got a different answer. Make sure to double check your numbers and use the correct formula to arrive at the correct answer.
 

1. What is the Doppler effect?

The Doppler effect is the change in frequency of a sound wave as the source of the sound moves relative to the observer. This results in a perceived change in pitch, with the frequency increasing as the source moves towards the observer and decreasing as the source moves away.

2. How is the Doppler effect used in everyday life?

The Doppler effect is used in a variety of everyday applications, such as police radar guns, weather radar, and medical imaging techniques like ultrasounds. It is also used in the design of musical instruments, such as sirens and car horns, to create a specific sound effect.

3. What is the difference between the Doppler effect and the Doppler shift?

The Doppler effect refers to the change in frequency of a sound wave, while the Doppler shift refers to the change in wavelength or speed of a wave. The two terms are often used interchangeably, but the Doppler shift can also refer to changes in other types of waves, such as light waves.

4. How does the speed of the source affect the Doppler effect?

The speed of the source affects the Doppler effect by determining the magnitude of the frequency shift. The faster the source is moving, the greater the change in frequency will be. This means that a car passing by at high speed will have a more noticeable change in sound frequency compared to a slow-moving car.

5. Does the Doppler effect only apply to sound waves?

No, the Doppler effect can also apply to other types of waves, such as light waves. However, it is most commonly observed and studied in relation to sound waves due to its practical applications and ease of demonstration in everyday life.

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