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Rotational Motion question

2025 · 23 Jan · Shift 2 · Q69
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Rotational Motion question

2025 · 23 Jan · Shift 2 · Q69

JEE MainPhysicsRotational MotionMCQ+4 / −1
A circular disk of radius R meter and mass M kg is rotating around the axis perpendicular to the disk. An external torque is applied to the disk such that θ(t)=5t2−8t\theta(t)=5 t^2-8 tθ(t)=5t2−8t, where θ(t)\theta(t)θ(t) is the angular position of the rotating disc as a function of time ttt. How much power is delivered by the applied torque, when t=2 st=2 \mathrm{~s}t=2 s ?
  1. A
    60MR260 \mathrm{MR}^260MR2
  2. B
    72MR272 \mathrm{MR}^272MR2
  3. C
    8MR28 \mathrm{MR}^28MR2
  4. D
    108MR2108 \mathrm{MR}^2108MR2
View written solutionFree

Correct answer: A

  1. Given angular position

    θ(t)=5t2−8t\theta(t)=5t^2-8tθ(t)=5t2−8t

  2. Find angular velocity and angular acceleration

    Angular velocity: ω=dθdt=10t−8\omega = \frac{d\theta}{dt} = 10t-8ω=dtdθ​=10t−8

    Angular acceleration: α=dωdt=10\alpha = \frac{d\omega}{dt} = 10α=dtdω​=10

  3. Evaluate at t=2 st=2\,\text{s}t=2s

    ω(2)=10(2)−8=12 rad/s\omega(2)=10(2)-8=12\,\text{rad/s}ω(2)=10(2)−8=12rad/s α=10 rad/s2\alpha=10\,\text{rad/s}^2α=10rad/s2

  4. Moment of inertia of the disk

    For a solid circular disk about an axis perpendicular to its plane through the center,

    I=12MR2I=\frac{1}{2}MR^2I=21​MR2

  5. Torque applied

    τ=Iα=12MR2⋅10=5MR2\tau = I\alpha = \frac{1}{2}MR^2 \cdot 10 = 5MR^2τ=Iα=21​MR2⋅10=5MR2

  6. Power delivered by torque

    Instantaneous power in rotational motion is

    P=τωP=\tau\omegaP=τω

    Therefore at t=2t=2t=2 s,

    P=5MR2×12=60MR2P = 5MR^2 \times 12 = 60MR^2P=5MR2×12=60MR2

  7. Match with options

    60MR2\boxed{60MR^2}60MR2​

    So the correct option is A.

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