Doppler Notes 2

A police car moves at a speed of 20ms-1 towards a listener who is moving towards the car at a speed of 3ms-1.
The speed of the sound is 325ms-1.
The police siren has a frequency of 400Hz.

1. Calculate the frequency the man hears. (430,16Hz)
2. Determine the Doppler Shift. (30,16Hz)

Hint.
The drawing is very important.
There are THREE VELOCITIES and TWO FREQUENCIES!
Study the layout carefully.

1. Draw a long arrow for the sound velocity, connecting the two source and listener.
2. Draw small arrows for each objects velocity.
3. The signs for the small arrows must be in accordance to the long arrow (long arrow = positive)
4. Small Velocity Arrow in same direction must be called positive.
5. Small Velocity Arrow in opposite direction must be called negative.
6. The frequencies are always positive.

Solution

The Doppler Shift is the DIFFERENCE (Subtraction) between the real and apparent frequencies.
430,16 400 = 30,16Hz
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