# 4 - Doppler's effect Questions Answers

when a tuning fork vibrates with 1.0 m or 1.05m long wire of a sonometer, 5 beats per second are produced in each case. what will be the frequency of the tuning fork ??

**Asked By: SARIKA SHARMA**

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**Joshi sir comment**

for sonometer wire

l = λ/2

or 2l = v/n here n represents frequency

or n = v/2l

for first wire of 1 m length n= v/2

for second wire of 1.05 m length n=v/2.1

let frequency of tuning fork = n_{1}

so according to the given condition v/2 = n_{1}+ 5

and v/2.1 = n_{1} - 5

solve now

in a large room, a person receives direct sound wave from a source 120m away from him. he also receives wave from the same source which reaches him after being reflected from 25m high ceiling at apoint midway between them. the two waves interfere constructively for wavelength of

(1) 20, 20/3, 4

(2) 10, 5, 3.33

(3) 10, 20, 30

(4) 15, 25, 35

**Asked By: HIMANSHU MITTAL**

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**Joshi sir comment**

path difference between two waves one coming directly (path length = 120 m) and other coming after reflection (path length = 130), you can get this by geometry

so path difference = 10 m

for constructive interference path difference should be a multiple of wavelength so (2) is answer

a source of sound and an observer are moving with velocities v = (2i + 4j)m/s and v = (6i - 3j)m/s. if frequency of source is 400 Hz, then frequency observed by observer will be

**Asked By: HIMANSHU MITTAL**

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**Joshi sir comment**

dot product of the two velocities given = 0, means observer and source are moving perpendicularly, so observer will observe the original frequency.

a source of sound of frequency f emits sound wave with speed v is at rest. if an observer is moving towards source with speed Vo(V not), then wavelength of sound waves observed by the observer will be

(1) v/f

(2) (v - Vo)/f

(3) [v(v - Vo)]/[f(v + Vo)]

(4) [v(v + Vo)]/[f(v - Vo)]

**Asked By: HIMANSHU MITTAL**

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**Joshi sir comment**

since velocity and frequency are changing proportionally so wavelength will be constant = v/f