Question

An ambulance with siren on, drives ina linear motion at constant speed. The frequency of the suren is f. You hear the frequency f'. For ech following question .Choose the correct answer and justify briefly:

1.You are on the road and the ambulance is getting closer to you. The frequecny f':

a) can change with respect to distance between you and the ambulance. b) is always greater than f. c) is always less than f. d) is first less than f at some point becomes greater than f.

Answer #1

In this case it has not been listed what is our velocity so i am assuming it to be zero. Now, it we start with the formula for doppler shift we have:

where c is the speed of sound waves in air.

Now velocity of source is constant and velocity of observer is zero which gives us:

The fraction on the right side will be less than 1 when the ambulance is moving towards us and as it moves away it becomes greater than 1.

This means that the frequency will first be less and after some point it becomes greater than f.

Therefore correct answer is option (d).

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