Doppler shifted frequency - ultrasound

1. Sep 8, 2008

bcjochim07

1. The problem statement, all variables and given/known data
An object is moving at speed vo toward a source at rest that is emitting sounds waves with frequency fo. What is the doppler shifted frequency of the reflected wave that comes back to the source?

2. Relevant equations

3. The attempt at a solution

If I use the doppler formula for an observer approaching a source : f+ = (1+vo/v) fo

This is the frequency seen by the moving object. But I'm not sure how to proceed. I think that the reflected wave would have a phase shift of pi, so does this question have something to with the superposition of the reflected waves and the waves coming from the source? I need a little nudge in the right direction.

Thanks

2. Sep 8, 2008

bcjochim07

any help would be greatly appreciated

3. Sep 9, 2008

bcjochim07

I think that the the object moving at the wave compresses it, thus increasing frequency, but I'm not sure how to express this mathematically.

4. Sep 10, 2008

alphysicist

Hi bcjochim07,

You've already found the frequency received by the moving object. For the reflected wave, the roles of source and observer are swapped. Do you see what to do now?