A stationary source is emitting sound at a fixed frequency f 0 , which is reflected by two cars approaching the source. The difference between the frequencies of sound reflected from the cars is 1.2 % of f 0 . What is the difference in the

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A stationary source is emitting sound at a fixed frequency f 0 , which is reflected by two cars approaching the source. The difference between the frequencies of sound reflected from the cars is 1.2 % of f 0 . What is the difference in the speeds of the cars (in km per hour) to the nearest integer ? The cars are moving at constant speeds much smaller than the speed of sound which is 330 ms -1 .

Quantrex Academy · Accepted Answer

Correct answer: 7

Let v 1 and v 2 be the velocities of sources, then apparent frequencies are f 1 = c + v 1 c - v 1 f 0 and f 2 = c + v 2 c - v 2 f 0 Then difference between frequencies is, ∆ f = 2 ∆ v C f 0 ∵ c > > v 1 & c > > v 2 ∴ ∆ v = 7 k m p h .

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