An ambulance is traveling East at 20m/s. The siren emits a fixed frequency of 27

Question

An ambulance is traveling East at 20m/s. The siren emits a fixed frequency of 270Hz. 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 in front of 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 has already passed the ambulance. A car driving West at a speed of 25m/s. The car is moving towards the ambulance.

Explanation / Answer

from the Doppler effect, an approaching sound apears as an increasing

frequency while a retreating sound appears as a decreasing frequency.

the order from highest to lowest frequency:

1. car drivng west at 15m/s

2. car sitting stationary   

3. car driving east at 20 m/s

4. car driving east at 25m/s

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