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Concepts Of Physics MCQ Edition [Volume 1]
Exam
Concepts Of Physics MCQ Edition [Volume 1]
Subject
Physics
Chapter
Rotational Mechanics

Concepts Of Physics MCQ Edition [Volume 1] · Physics · Rotational Mechanics

A hollow sphere of radius R rests on a smooth horizontal surface....

A hollow sphere of radius RR rests on a smooth horizontal surface. It is pulled by a horizontal force acting tangentially at the highest point. Determine the distance travelled by the sphere during the time it completes one full rotation.

Options

  1. AπR\pi R
  2. B2πR2\pi R
  3. C4πR3\dfrac{4\pi R}{3}
  4. D4πR5\dfrac{4\pi R}{5}

Correct answer

C. 4πR3\dfrac{4\pi R}{3}

Step-by-step solution

Let the mass of the hollow sphere be mm and the applied horizontal force be FF.

The linear acceleration of the center of mass is given by Newton's second law:

a=Fma = \dfrac{F}{m}

The torque acting about the center of mass due to the force at the highest point is:

τ=FR\tau = F R

The moment of inertia of a hollow sphere about its center of mass is I=23mR2I = \dfrac{2}{3}mR^2. The angular acceleration α\alpha is:

α=τI=FR23mR2=3F2mR\alpha = \dfrac{\tau}{I} = \dfrac{FR}{\dfrac{2}{3}mR^2} = \dfrac{3F}{2mR}

Using the kinematic equations for linear and angular motion starting from rest, the distance travelled ss and the angular displacement θ\theta in time tt are:

s=12at2s = \dfrac{1}{2}at^2

θ=12αt2\theta = \dfrac{1}{2}\alpha t^2

Taking the ratio of ss to θ\theta:

sθ=aα=Fm3F2mR=2R3\dfrac{s}{\theta} = \dfrac{a}{\alpha} = \dfrac{\dfrac{F}{m}}{\dfrac{3F}{2mR}} = \dfrac{2R}{3}

For one full rotation, the angular displacement is θ=2π\theta = 2\pi. Substituting this into the ratio gives the distance travelled:

s=2R3×2π=4πR3s = \dfrac{2R}{3} \times 2\pi = \dfrac{4\pi R}{3}
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