The Doppler Effect and Sound

Question

A 500Hz whistle is moved away from a listener at a speed of 10m/s. At the same time, the listener moves at a speed of 20m/s in the direction of the whistle. What is the apparent frequency heard by the listener? The veocity of Sound in air is 340 m/s.

Here is how I worked it out. Please tell me if I did something wrong.

Fo=Fs(V-Vo/V-Vs)

Fs= 500Hz

V=340 m/s

Vo= 20m/s (It is postive because is moving toward the whistle at a higher velocity than the whistle is moving away.)

Vs=-10m/s (It is negative because it is moving away from the observer.)

Fo=500((340 20)/(340-(-10)))

Fo=500(360/350)

Fo=514Hz

Is this right? Its the signs for Vs and Vo where I am a little shakey.

Answer 1

Since the combined velocities result in the observer approaching the source, the observed frequency should be higher than the frequency at the source, so your answer is reasonable.