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. Determine if the driver of the car will hear a frequency the same as 270Hz, higher than 270Hz, or lower than 270Hz.

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

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

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

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

Answer 1

Solution,

According to Doppler effect, an approaching sound apears as an increasing frequency while a retreating sound appears as a decreasing frequency.

A)

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 270 Hz

B)

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 270 Hz

C)

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 270 Hz

D)

For a stationary car watchiing the ambulance moving away, the frequency would appear to decrease and therefore seem less than 270 Hz

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