MCQMediumJEE 2024Simple Harmonic Motion (SHM)

JEE Physics 2024 Question with Solution

A simple harmonic oscillator has an amplitude AA and a time period 6π6\pi seconds. Assuming the oscillation starts from its mean position, the time required by it to travel from x=Ax = A to x=32Ax = \frac{\sqrt{3}}{2} A will be πx\frac{\pi}{x} seconds, where xx is:

  • A

    33

  • B

    55

  • C

    22

  • D

    44

Answer

Correct answer:C

Step-by-step solution

Standard Method

Given: Amplitude is AA, time period is T=6πsT = 6\pi \, \text{s}, and the oscillator starts from the mean position.

Find: The value of xx if the time taken to travel from x=Ax = A to x=32Ax = \frac{\sqrt{3}}{2}A is πx\frac{\pi}{x} seconds.

For SHM starting from the mean position, we can use the reference-circle relation. When the particle moves from the extreme position x=Ax = A to x=32Ax = \frac{\sqrt{3}}{2}A, we write

xA=cosθ=32\frac{x}{A} = \cos\theta = \frac{\sqrt{3}}{2}

so

θ=π6.\theta = \frac{\pi}{6}.

Hence the corresponding time interval satisfies

ωt=π6.\omega t = \frac{\pi}{6}.

Using

ω=2πT,\omega = \frac{2\pi}{T},

we get

2πTt=π6.\frac{2\pi}{T} \, t = \frac{\pi}{6}.

Therefore,

t=T12.t = \frac{T}{12}.

Since T=6πT = 6\pi seconds,

t=6π12=π2  s.t = \frac{6\pi}{12} = \frac{\pi}{2} \; \text{s}.

Now compare with the given form

t=πx.t = \frac{\pi}{x}.

So,

πx=π2.\frac{\pi}{x} = \frac{\pi}{2}.

Thus,

x=2.x = 2.

Therefore, the correct option is C.

Reference Circle Interpretation

Given: T=6πsT = 6\pi \, \text{s} and displacement changes from AA to 32A\frac{\sqrt{3}}{2}A.

Find: The value of xx in t=πxt = \frac{\pi}{x}.

At the extreme position, the SHM phase for cosine representation is 00. Moving to displacement 32A\frac{\sqrt{3}}{2}A means

cosθ=32.\cos\theta = \frac{\sqrt{3}}{2}.

This gives

θ=π6.\theta = \frac{\pi}{6}.

The time for a phase change θ\theta is

t=θω=π/6ω.t = \frac{\theta}{\omega} = \frac{\pi/6}{\omega}.

Now,

ω=2πT=2π6π=13rad/s.\omega = \frac{2\pi}{T} = \frac{2\pi}{6\pi} = \frac{1}{3} \, \text{rad/s}.

So,

t=π/61/3=π2  s.t = \frac{\pi/6}{1/3} = \frac{\pi}{2} \; \text{s}.

Thus,

πx=π2\frac{\pi}{x} = \frac{\pi}{2}

which gives

x=2.x = 2.

Therefore, the correct option is C. The solution also concludes x=2x = 2.

Common mistakes

  • Using the displacement from the mean position directly without noticing that the asked interval is from x=Ax = A to x=32Ax = \frac{\sqrt{3}}{2}A. This changes the phase interval. Instead, measure the phase change from the extreme position for the required travel.

  • Confusing angular frequency with time period. Writing ω=1T\omega = \frac{1}{T} is incorrect because the correct relation is ω=2πT\omega = \frac{2\pi}{T}. Always include the factor 2π2\pi.

  • Taking cosθ=32\cos\theta = \frac{\sqrt{3}}{2} and choosing the wrong angle such as θ=5π6\theta = \frac{5\pi}{6}. For the first travel from the extreme position, the relevant smallest positive phase is π6\frac{\pi}{6}.

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