doppler effect

[swamprat]

Can someone please explain a way to know what sign to use in the equation f'=f(v+-Vd)/(v+-Vs)

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[futuredoctor10]

Sure!

f ' = f (v +- Vd) / (v -+ Vs)

+ = detector moves toward source
- = detector moves away from source

- = source moves toward detector
+ = source moves away from detector

How to Learn this Concept with Less Memorization!
The observed frequency f ' will increase if the 2 things (detector/source) move toward each other.

The observed frequency f ' will decrease if they (detector/source) move apart.

If the source moves toward the detector, observed frequency should INCREASE.
For this to happen, you must use v - Vs in the denominator of the above equation.
By decreasing the denominator you increase the observed frequency.

Make sense?

 

[swamprat]

beautiful thank you very much

 

[Vihsadas]

Sure!

f ' = f (v +- Vd) / (v -+ Vs)

+ = detector moves toward source
- = detector moves away from source

- = source moves toward detector
+ = source moves away from detector

How to Learn this Concept with Less Memorization!
The observed frequency f ' will increase if the 2 things (detector/source) move toward each other.

The observed frequency f ' will decrease if they (detector/source) move apart.

If the source moves toward the detector, observed frequency should INCREASE.
For this to happen, you must use v - Vs in the denominator of the above equation.
By decreasing the denominator you increase the observed frequency.

Make sense?

That is the perfect way to think about the problem. GJ. We should be looking for relationships analogous to this one in other sections of the PS and BS.
Ideally, you'd memorize the rules, but then if you get a question on the concepts, because you also know how to work through it like futuredoc did, you'd do okay at that too.

 

[futuredoctor10]

Thanks! I hope and believe all the studying pays off!

And that's true- knowing the rules makes things faster, but keep how to find those rules (working it through) as backup!