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JEE Main 2021 (22 Jul, Shift 2) · Spring Systems

Spring Systems question from JEE Main 2021 (22 Jul, Shift 2)

Q69JEE Main 2021 · 22 JUL · SHIFT 2Medium

Spring Systems

The motion of a mass on a spring, with spring constant K is as shown in figure.

Figure

The equation of motion is given by

x(t) = A sinω\omegat + B cosω\omegat with ω=Km\omega = \sqrt{\frac{K}{m}}

Suppose that at time t = 0, the position of mass is x(0) and velocity v(0), then its displacement can also be represented as x(t) = C cos(ω\omegat −ϕ-\phi), where C and ϕ\phi are:

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Answer: (c)

  1. x(t)=Ccos⁡(ωt−ϕ)x(t) = C\cos(\omega t - \phi), so v(t)=−Cωsin⁡(ωt−ϕ)v(t) = -C\omega\sin(\omega t - \phi).
  2. At t=0t = 0: x(0)=Ccos⁡ϕx(0) = C\cos\phi and v(0)=Cωsin⁡ϕv(0) = C\omega\sin\phi.
  3. Square and add: C=x(0)2+v(0)2ω2C = \sqrt{x(0)^2 + \frac{v(0)^2}{\omega^2}}. Divide: tan⁡ϕ=v(0)x(0)ω\tan\phi = \frac{v(0)}{x(0)\omega}.
  4. So the answer is (c).
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