An ambulance is traveling East at 20m/s. The siren emits a fixed frequency of 39

Question

An ambulance is traveling East at 20m/s. The siren emits a fixed frequency of 390Hz. 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 2 3 4  A car driving East at a speed of 25m/s. The car is ahead of the ambulance.
1 2 3 4  A car driving East at a speed of 15m/s. The car is ahead of the ambulance.
1 2 3 4  A car driving East at a speed of 20m/s. The car is behind the ambulance.
1 2 3 4  A car sitting stationary at the side of the road watching the ambulance moving away.

Explanation / Answer

f s = 390 Hz, vs = 20 m/s

vo = 25 m/s

Velocity of sound in air v = 343 m/s

From doppler effect fo = ((v-vo)/(v-vs))f

fo = ((343 -25)/(343- 20)*390

fo = 383.9 Hz

2) fo = ((343 +15)/(343-20)*390

fo = 432 Hz

3) fo = ((343 + 20)/(343- 20)*390

fo =438.3 Hz

4) fo = (343/(343-20))*390

fo =414 Hz

3 > 2 > 4> 1

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