An ambulance is traveling East at 20m/s. The siren emits a fixed frequency of 41

Question

An ambulance is traveling East at 20m/s. The siren emits a fixed frequency of 410Hz. 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 410Hz, higher than 410Hz, or lower than 410Hz.

Higher, Lower, or Same

A car sitting stationary at the side of the road watching the oncoming ambulance.
A car driving East at a speed of 20m/s. The car is behind the ambulance.
A car driving West at a speed of 15m/s. The car is moving towards the ambulance.
A car driving West at a speed of 25m/s. The car is moving towards the ambulance.

Explanation / Answer

Here ,

speed of ambulance = 20 m/s east

frequency at source , fo = 410 Hz

for the car sitting when the ambulance is coming

as the ambulance is appraoching ,

the car will hear more than 410 Hz

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Now, for the car driving in the east direction ,

as the relative speed = 20 - 20

relative speed = 0 m/s

the driver will hear same as 410 Hz
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for the driver going west

as the car and ambulance is moving away from each other

the car will hear less than 410 Hz
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for the car to approach towards the ambulance at 25 m/s

as the car is coming closer to ambulance ,

car will hear frequency higher than 410 Hz

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