An ambulance is traveling East at 20m/s. The siren emits a fixed frequency of 44

Question

An ambulance is traveling East at 20m/s. The siren emits a fixed frequency of 440Hz. Each of the following cars is traveling down the road as described. Rank the cars according to the frequency the driver will hear. Select 1 for the car which will hear the highest frequency. Remember that if the frequency is the same, select the same rank and skip the next number. I really suggest drawing a picture to help sort this out. Keep in mind the relative speed - How fast does it seem the ambulance is moving towards or away from each driver?

A car driving East at a speed of 20m/s. The car is behind the ambulance.

A car sitting stationary at the side of the road watching the oncoming ambulance.

A car driving West at a speed of 15m/s. The car is moving towards the ambulance.

A car driving West at a speed of 25m/s. The car is moving towards the ambulance.

Explanation / Answer

frequency shift due to doppler effect is given by,
f/ = f (v +/- vO) / (v -/+ vs )

v = 343 m/s
f = 440 Hz
f' is the observed freq,

1st Case,
Relative speed = 0 btw Ambulance & Car
f' = 440 Hz

2nd case,
f' = 440 * (343)/ (343 - 20)
f' = 467.2 Hz

3rd Case,
f' = 440 * (343 + 15 )/ (343 - 20)
f' = 487.6 Hz

4th Case,
f' = 440 * (343 + 25 )/ (343 - 20)
f' = 501.3 Hz

4th Case will hear the highest freq.

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