Calculating frequencies and velocity

[catfish]

[Mentor's note - This thread was moved here from a non-homework forum, therefore it does not use the template.]

Hi guys

Need some help answering questions about frequencies and calculating velocity.
I'm given two frequencies: 470Hz for an approaching object, 440Hz for when the object is moving away. Velocity of the speed of sound is 343m/s. Work out the frequency of the sound at the object. The receiver is stationary. What formula do I use to work this out?

And once that's been calculated, can velocity be calculated using this formula? fr = fs(v/(v – vs)) where fr is frequency heard by the receiver, fs is frequency from the sender, v is velocity of sound through air, and vs is velocity of the object.
Any help is appreciated.

 

[billy_joule]

I think this belongs in the homework and coursework part of the forum, but anyway..

The first equation here:

http://en.wikipedia.org/wiki/Doppler_effect

plug in you data for the two cases (coming and going), you'll have a pair of linear equations, solve them for the two unknowns (Velocity of source and emitted frequency).


There is a much faster, intuitive way of getting emitted frequency ( and then Vs) which will hopefully become obvious when you see it's value after calculating it the longer way...

 

[catfish]

I've got an answer for my first question. Is it supposed to be 455Hz?

 

[acegikmoqsuwy]

catfish, I think we also need to know if the speed of the object when it's moving away is equal or not to the speed when the object is approaching.

 

[catfish]

I think it's assumed that it is but I apparently also have to work out the speed of the object using the information and the formula provided.

 

[billy_joule]

I've got an answer for my first question. Is it supposed to be 455Hz?

Yes, it's just the average of the coming and going frequency: (470+440)/2

Now just plug some numbers in and solve for Vs

 

[catfish]

Sweet, thanks for the help.