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Rotational Motion question

2019 · Shift 2 · Q41
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  5. /2019 · Shift 2 · Q41

Rotational Motion question

2019 · Shift 2 · Q41

JEE AdvancedPhysicsRotational MotionMultiple correct+4 / −1
A thin and uniform rod of mass M and length L is held vertical on a floor with large friction. The rod is released from rest so that it falls by rotating about its contact-point with the floor without slipping. Which of the following statement(s) is/are correct, when the rod makes an angle 60 ∘^\circ∘ with vertical? [g is the acceleration due to gravity]
  1. A
    The angular acceleration of the rod will be 2gL{{2g} \over L}L2g​.
  2. B
    The normal reaction force from the floor on the rod will be Mg16{{Mg} \over 16}16Mg​.
  3. C
    The radial acceleration of the rod's center of mass will be 3g4{{3g} \over 4}43g​.
  4. D
    The angular speed of the rod will be 3g2L\sqrt {{{3g} \over {2L}}}2L3g​​.
View written solutionFree

Correct answer: B, C, D

  1. Model the motion

Since the floor has large friction, the lower end of the rod does not slip. So the rod rotates about the contact point with the floor as a fixed pivot.

For a uniform rod of length LLL pivoted at one end:

  • Moment of inertia about the pivot: IO=13ML2I_O = \frac{1}{3}ML^2IO​=31​ML2
  • Distance of center of mass from pivot: rCM=L2r_{CM} = \frac{L}{2}rCM​=2L​

Let θ\thetaθ be the angle the rod makes with the vertical. We are asked for θ=60∘\theta = 60^\circθ=60∘.


  1. Angular speed using energy conservation

Initially, the rod is vertical and at rest.

When the rod makes angle θ\thetaθ with the vertical, the center of mass has fallen by Δh=L2−L2cos⁡θ=L2(1−cos⁡θ)\Delta h = \frac{L}{2} - \frac{L}{2}\cos\theta = \frac{L}{2}(1-\cos\theta)Δh=2L​−2L​cosθ=2L​(1−cosθ)

Loss in gravitational potential energy = gain in rotational kinetic energy: Mg⋅L2(1−cos⁡θ)=12IOω2Mg\cdot \frac{L}{2}(1-\cos\theta) = \frac{1}{2}I_O\omega^2Mg⋅2L​(1−cosθ)=21​IO​ω2

Substitute IO=13ML2I_O = \frac{1}{3}ML^2IO​=31​ML2: Mg⋅L2(1−cos⁡θ)=12⋅13ML2ω2Mg\cdot \frac{L}{2}(1-\cos\theta) = \frac{1}{2}\cdot \frac{1}{3}ML^2\omega^2Mg⋅2L​(1−cosθ)=21​⋅31​ML2ω2

MgL2(1−cos⁡θ)=16ML2ω2\frac{MgL}{2}(1-\cos\theta) = \frac{1}{6}ML^2\omega^22MgL​(1−cosθ)=61​ML2ω2

3g(1−cos⁡θ)=Lω23g(1-\cos\theta)=L\omega^23g(1−cosθ)=Lω2

At θ=60∘\theta=60^\circθ=60∘, cos⁡60∘=12\cos60^\circ=\frac12cos60∘=21​: Lω2=3g(1−12)=3g2L\omega^2 = 3g\left(1-\frac12\right)=\frac{3g}{2}Lω2=3g(1−21​)=23g​

So, ω2=3g2L\omega^2=\frac{3g}{2L}ω2=2L3g​ ω=3g2L\omega=\sqrt{\frac{3g}{2L}}ω=2L3g​​

Therefore, Option D is correct.


  1. Angular acceleration

Torque of gravity about the pivot is τ=Mg⋅L2sin⁡θ\tau = Mg\cdot \frac{L}{2}\sin\thetaτ=Mg⋅2L​sinθ

Using τ=IOα\tau = I_O\alphaτ=IO​α: Mg⋅L2sin⁡θ=13ML2αMg\cdot \frac{L}{2}\sin\theta = \frac{1}{3}ML^2\alphaMg⋅2L​sinθ=31​ML2α

α=3g2Lsin⁡θ\alpha = \frac{3g}{2L}\sin\thetaα=2L3g​sinθ

At θ=60∘\theta=60^\circθ=60∘: α=3g2L⋅32=334gL\alpha = \frac{3g}{2L}\cdot \frac{\sqrt3}{2} = \frac{3\sqrt3}{4}\frac{g}{L}α=2L3g​⋅23​​=433​​Lg​

This is not equal to 2gL\frac{2g}{L}L2g​.

Therefore, Option A is incorrect.


  1. Radial acceleration of the center of mass

For rotation about a fixed point, radial (centripetal) acceleration of the center of mass is ar=ω2(L2)a_r = \omega^2\left(\frac{L}{2}\right)ar​=ω2(2L​)

Using ω2=3g2L\omega^2 = \frac{3g}{2L}ω2=2L3g​: ar=3g2L⋅L2=3g4a_r = \frac{3g}{2L}\cdot \frac{L}{2} = \frac{3g}{4}ar​=2L3g​⋅2L​=43g​

Therefore, Option C is correct.


  1. Normal reaction from the floor

Let us find acceleration of the center of mass in vertical direction.

For a rod making angle θ\thetaθ with vertical, taking pivot as origin, x=L2sin⁡θ,y=L2cos⁡θx = \frac{L}{2}\sin\theta, \qquad y = \frac{L}{2}\cos\thetax=2L​sinθ,y=2L​cosθ

Differentiate twice: y¨=−L2(αsin⁡θ+ω2cos⁡θ)\ddot y = -\frac{L}{2}(\alpha\sin\theta + \omega^2\cos\theta)y¨​=−2L​(αsinθ+ω2cosθ)

At θ=60∘\theta=60^\circθ=60∘:

  • sin⁡60∘=32\sin60^\circ = \frac{\sqrt3}{2}sin60∘=23​​
  • cos⁡60∘=12\cos60^\circ = \frac12cos60∘=21​
  • α=334gL\alpha = \frac{3\sqrt3}{4}\frac{g}{L}α=433​​Lg​
  • ω2=3g2L\omega^2 = \frac{3g}{2L}ω2=2L3g​

So, ay=y¨=−L2(334gL⋅32+3g2L⋅12)a_y = \ddot y = -\frac{L}{2}\left(\frac{3\sqrt3}{4}\frac{g}{L}\cdot \frac{\sqrt3}{2} + \frac{3g}{2L}\cdot \frac12\right)ay​=y¨​=−2L​(433​​Lg​⋅23​​+2L3g​⋅21​)

ay=−L2(9g8L+3g4L)a_y = -\frac{L}{2}\left(\frac{9g}{8L} + \frac{3g}{4L}\right)ay​=−2L​(8L9g​+4L3g​)

ay=−L2(15g8L)=−15g16a_y = -\frac{L}{2}\left(\frac{15g}{8L}\right) = -\frac{15g}{16}ay​=−2L​(8L15g​)=−1615g​

Now apply Newton's second law in vertical direction: N−Mg=MayN - Mg = M a_yN−Mg=May​

N−Mg=−15Mg16N - Mg = -\frac{15Mg}{16}N−Mg=−1615Mg​

N=Mg−15Mg16=Mg16N = Mg - \frac{15Mg}{16} = \frac{Mg}{16}N=Mg−1615Mg​=16Mg​

Therefore, Option B is correct.


  1. Final evaluation of options
  • A: Incorrect
  • B: Correct
  • C: Correct
  • D: Correct

So the correct choices are: B, C, D\boxed{B,\ C,\ D}B, C, D​


  1. Comparison with stored correct answer

Stored correct answer: B, C, D

My derived answer: B, C, D

They match exactly.

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