Question

A person sitting in a parked car hears an approaching ambulance siren at a frequency f1. as it passes him and moves away, he hears a frequency f2. the actual frequency f of the source is (which one of the following)

a. f > f1
b. f < f2
c.f= f2 - f1
d. f = f2 + f1
e. f2 < f < f1

Answer 1

Answer:E

Explanation:

This can be explained by Doppler effect which gives the relation between apparent Frequency and actual frequency when the source of sound is moving

$f'=f\frac{v+v_o}{v-v_s}$

where $f'=apparent\ frequency$

$f=actual\ frequency$

$v=velocity\ of\ sound$

$v_o=velocity\ of\ observer$

$v_s=velocity\ of\ source$

here $v_o =0$ as observer is standing

when Ambulance is approaching then velocity of sound and velocity of ambulance have relative velocity thus denominator is less than and apparent frequency is more while when ambulance is going away then velocity of sound waves and velocity of observer is in opposite direction thus denominator is less than 1 and apparent frequency is less.

Thus $f_2<f<f_1$