(You know, you would think with 2 and 1/2 degrees in Physics under my belt I'd have the Doppler equations memorized, wouldn't you? )Originally Posted byCelia

For a source moving toward an object

where f' is the observed frequency, f is the frequency of the source measured when it isn't moving, v is the speed of sound, and v0 is the speed that the observer is moving toward the source. (We are taking the source to be stationary.)

According to the problem statement f' is 5% larger than f, so f' = 1.05f. Thus:

The f's cancel and you can solve for v0. NOTE: Your answer will be in m/s, so you have to convert to km/h from here.

-Dan