Wave Motion 1 Question 2

2. A submarine A travelling at 18km/h is being chased along the line of its velocity by another submarine B travelling at 27km/h.B sends a sonar signal of 500Hz to detect A and receives a reflected sound of frequency v. The value of v is close to (Speed of sound in water =1500ms1 )

(2019 Main, 12 April I)

(a) 504Hz

(b) 507Hz

(c) 499Hz

(d) 502Hz

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Answer:

Correct Answer: 2. (d)

Solution:

  1. Given, velocity of submarine (A),

vA=18km/h=180003600m/s or vA=5m/s

and velocity of submarine (B),

vB=27km/h=270003600m/s

or vB=7.5m/s

Signal sent by submarine (B) is detected by submarine (A) can be shown as

Frequency of the signal, fo=500Hz

So, in this relative motion, frequency received by submarine (A) is

f1=(vSvAvSvB)fo=(1500515007.5)500Hzf1=14951492.5×500Hz

The reflected frequency f1 is now received back by submarine (B).

So, frequency received at submarine (B) is

f2=(vS+vBvS+vA)f1=(1500+7.51500+5)(14951492.5)500Hzf2=(1507.51505)(14951492.5)500Hzf2=1.00166×1.00167×500f2=501.67Hz502Hz



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