An ambulance is traveling East at 20m/s. The siren emits a fixed frequency of 36

Question

An ambulance is traveling East at 20m/s. The siren emits a fixed frequency of 360Hz. Each of the following cars is traveling down the road as described. Determine if the driver of the car will hear a frequency the same as 360Hz, higher than 360Hz, or lower than 360Hz. I really suggest drawing a picture to help sort this out.

1. A car driving West at a speed of 25m/s. The car has already passed the ambulance.

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

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

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

Explanation / Answer

In cases of frequency in motion, as per the Doppler effect, an approaching sound apears as an increasing frequency while a retreating sound appears as a decreasing frequency. Thus the following would be the effects experienced for the following cases:

1) The car is moving West and away from the ambulance (which is moving East) and hence the the frequency would appear to decrease and be less than 360Hz

2) The car is driving West but towards the ambulance. In this case since the sound is approaching it would appear at an increasing frequency and above 360Hz.

3) The car is driving behind the ambulance and at the same speed. Therefore, in this case, there is no relative motion and hence the frequency remains constant at 360Hz

4) For a stationary car watchiing the ambulance moving away, the frequency would appear to decrease and therefore seem less than 360Hz

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