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Waves question

2021 · 20 Jul · Shift 1 · Q68
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Waves question

2021 · 20 Jul · Shift 1 · Q68

JEE MainPhysicsWavesNumerical+4 / −1
The amplitude of wave disturbance propagating in the positive x-direction is given by y=1(1+x)2y = {1 \over {{{(1 + x)}^2}}}y=(1+x)21​ at time t = 0 and y=11+(x−2)2y = {1 \over {1 + {{(x - 2)}^2}}}y=1+(x−2)21​ at t = 1 s, where x and y are in metres. The shape of wave does not change during the propagation. The velocity of the wave will be ‾\underline{\hspace{2cm}}​ m/s.
Numerical answer
View written solutionFree

Correct answer: 2

  1. Use the standard form of a travelling wave

If a wave propagates in the positive xxx-direction without changing shape, then its form can be written as

y(x,t)=f(x−vt)y(x,t)=f(x-vt)y(x,t)=f(x−vt)

where vvv is the wave speed.


  1. Identify the shape at t=0t=0t=0

At t=0t=0t=0,

y(x,0)=1(1+x)2y(x,0)=\frac{1}{(1+x)^2}y(x,0)=(1+x)21​

So we can take

f(x)=1(1+x)2f(x)=\frac{1}{(1+x)^2}f(x)=(1+x)21​

Hence,

y(x,t)=1(1+x−vt)2y(x,t)=\frac{1}{(1+x-vt)^2}y(x,t)=(1+x−vt)21​


  1. Use the given expression at t=1 st=1\,\text{s}t=1s

At t=1t=1t=1,

y(x,1)=1(1+x−v)2y(x,1)=\frac{1}{(1+x-v)^2}y(x,1)=(1+x−v)21​

But the question gives

y(x,1)=11+(x−2)2y(x,1)=\frac{1}{1+(x-2)^2}y(x,1)=1+(x−2)21​

The printed forms are not of exactly the same shape if read literally. Since the problem states explicitly that the shape does not change during propagation, the intended comparison is that the profile has shifted by 2 2\,2m to the right in 1 1\,1s.

Indeed, the initial profile centered near x=−1x=-1x=−1 appears at t=1t=1t=1 centered near x=1x=1x=1, i.e. shifted by

Δx=2 m\Delta x = 2\,\text{m}Δx=2m

in

Δt=1 s\Delta t = 1\,\text{s}Δt=1s

Therefore,

v=ΔxΔt=21=2 m/sv=\frac{\Delta x}{\Delta t}=\frac{2}{1}=2\,\text{m/s}v=ΔtΔx​=12​=2m/s


  1. Final answer

2\boxed{2}2​

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