An ambulance is traveling East at 20m/s. The siren emits a fixed frequency of 42

Question

An ambulance is traveling East at 20m/s. The siren emits a fixed frequency of 420Hz. 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?

1)A car driving East at a speed of 15m/s. The car is behind the ambulance.

2)A car sitting stationary at the side of the road watching the ambulance moving away.

3)A car driving East at a speed of 20m/s. The car is in front of the ambulance.

4)A car driving East at a speed of 25m/s. The car is ahead of the ambulance.

Explanation / Answer

1) f=f0*(u+15)/(u+20) = 0.986 f0

2) f=f0*u/(u+20) = 0.945 f0

3) f=f0

4) f=f0*(u-25)/(u-20) = 0.985 f0

therefore: 3)>1)>4)>2

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