Doppler effect Indianapolis 500 problem

In summary: So the car is going 343 m/s when it is coming towards you and it is going 611 m/s when it is moving away from you.
  • #1
RedHotLlama18
1
0

Homework Statement



At the Indianapolis 500, you can measure the speed of cars just by listening to the difference in pitch of the engine noise between approaching and receding cars. Suppose the sound of a certain car drops by a factor of 1.50 as it goes by on the straightaway. How fast is it going? (Take the speed of sound to be 343 m/s.)

answer given in km/h

Homework Equations




v= (1- (f/(f prime/1.5))v

where the second v is the speed of sound.

Sorry it isn't a cleaner equation, I couldn't figure out how to get my mathtype equation from microsoft word here.




The Attempt at a Solution


The equation above is my attempt at a solution, since the sound of the cars frequency drops by 1.5 that's what I divided my f prime by. I just don't know how to figure out the f values. Maybe I am using the wrong equation, I just can't figure out any other way to solve for the speed of the car. Any hints would be greatly appreciated and I thank you for your time.
 
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  • #2
1.5 IS f/f'. You don't need to know the values of f.
 
  • #3
And be careful. When the car is coming toward you it's doppler shifted up in pitch and when it leaves you it is shifted down. As I read the question there is a factor of 1.5 between the ratios of those two pitches.
 
  • #4
RedHotLlama18 said:

Homework Statement



At the Indianapolis 500, you can measure the speed of cars just by listening to the difference in pitch of the engine noise between approaching and receding cars. Suppose the sound of a certain car drops by a factor of 1.50 as it goes by on the straightaway. How fast is it going? (Take the speed of sound to be 343 m/s.)

answer given in km/h

Homework Equations




v= (1- (f/(f prime/1.5))v

where the second v is the speed of sound.

Sorry it isn't a cleaner equation, I couldn't figure out how to get my mathtype equation from microsoft word here.




The Attempt at a Solution


The equation above is my attempt at a solution, since the sound of the cars frequency drops by 1.5 that's what I divided my f prime by. I just don't know how to figure out the f values. Maybe I am using the wrong equation, I just can't figure out any other way to solve for the speed of the car. Any hints would be greatly appreciated and I thank you for your time.

Welcome to the forums!

I don't quite see how you got your equation but I have a feeling that you used incorrectly the factor of 1.5.

Set up an equation for the frequency of the sound when the car is going toward you. Call this frequency [itex] f^' [/itex]. It will contain the frequency emitted by the source that I will call [itex] f_{source} [/itex] to be clear. Now find an expression for the frequency when the car is moving away from you. Let's call this f double prime = [itex] f^{''}[/itex].

What they are telling you is that [itex] \frac{f^'}{f^{''}} = 1.5 [/itex]. So take the ratio of your two equations and you will see that [itex] f_{source}[/itex] will drop out, leaving you with an equation for the speed of the source (the car) in terms of the speed of sound.
 

What is the Doppler effect Indianapolis 500 problem?

The Doppler effect Indianapolis 500 problem is a physics problem that involves the change in frequency of sound waves as a source moves towards or away from an observer. This is commonly seen in situations such as a race car passing by a stationary observer, where the sound of the car's engine appears to change in pitch as it approaches and then passes by.

How does the Doppler effect affect the sound of cars at the Indianapolis 500?

The Doppler effect causes the sound of cars at the Indianapolis 500 to appear to change in pitch as they move towards and away from the audience. As the cars approach, the sound waves are compressed and the pitch appears to be higher. As they pass by, the sound waves are stretched and the pitch appears to be lower.

What factors can affect the perceived Doppler effect at the Indianapolis 500?

The perceived Doppler effect at the Indianapolis 500 can be affected by the speed of the cars, the distance between the cars and the audience, and the angle at which the cars are passing by. The closer the cars are to the audience and the faster they are traveling, the more noticeable the Doppler effect will be.

How is the Doppler effect used in other areas besides the Indianapolis 500?

The Doppler effect is used in various fields such as astronomy, meteorology, and medical imaging. In astronomy, it is used to determine the movement of stars and galaxies. In meteorology, it is used to track the movement of storms. In medical imaging, it is used to create images of organs and tissues inside the body.

Is the Doppler effect always perceived as a change in pitch?

No, the Doppler effect can also be perceived as a change in frequency or wavelength. In situations where the source is moving perpendicular to the observer, the frequency or wavelength may appear to be higher or lower, rather than the pitch. This is commonly seen in situations such as the movement of objects in a circular motion or the rotation of a pulsating star.

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