Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Doppler question



On Tue, 17 Mar 1998, Tony Wayne wrote:

Here's a doppler question.

I always thought motion was relative. Why is it if the source
emits a 440 hz and it moves towards the observer at 30 m/s, the
observer hears 482 hz.(c=345). But if the observer moves
towards a 440 hz sound source at 30 m/s, the observer hears 478
hz. Why are these two heard frequencies different? In both
questions the observer and the source are approaching each
other. What *conceptually* is going on here?

I assume that you mean to say that, in the first case, the *source* moves
relative to the medium (air) and the observer doesn't while, in the second
case, it is the *observer* who moves relative to the air. Since the two
cases are demonstrably different as seen by all observers, there should be
no concern that the observed frequencies are different.

Now try doing this exercise for electromagnetic waves and see what you
come up with!

John
-----------------------------------------------------------------
A. John Mallinckrodt http://www.intranet.csupomona.edu/~ajm
Professor of Physics mailto:ajmallinckro@csupomona.edu
Physics Department voice:909-869-4054
Cal Poly Pomona fax:909-869-5090
Pomona, CA 91768-4031 office:Building 8, Room 223