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Moment Of Inertia Of A Sphere

Moment Of Inertia Of A Sphere

The Moment of Inertia (MI) is a property of a body that measures its resistance to rotational motion about a given axis. A solid sphere is a three-dimensional object where the mass is uniformly distributed throughout its volume. Due to its perfect symmetry, the moment of inertia of a sphere is the same about any axis passing through its centre.

Moment of Inertia About Different Axes

Considering a solid sphere of mass M and radius R. The following are cases for the moment of inertia of a sphere.

Moment of Inertia About a Diameter

The sphere is divided into a stack of thin circular discs perpendicular to the axis of rotation. The radius of each disc changes depending on its position along the axis.

Derivation:

Volume Element

At a distance x from the centre, the radius of the cross-section is:

formula

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Moment of Inertia About a Tangent

formula

Applications

  • Planetary rotation – Planets are approximated as solid spheres for spin calculations.
  • Sports – Motion of balls like cricket, football, and baseball.
  • Engineering – Flywheels and ball bearings.
  • Astronomy – Rotation of stars modelled as solid spheres.

Summing Up

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Frequently Asked Questions

Q1. Why is the moment of inertia of a solid sphere smaller than that of a hollow sphere?

The moment of inertia is higher for a hollow sphere. Because in a solid sphere, more mass is concentrated closer to the axis, reducing resistance to rotation.

Q2. What role does symmetry play for the moment of inertia in a sphere?

Symmetry ensures that the moment of inertia is the same in all directions through the centre.

Q3. A solid sphere has a mass of 6 kg and a radius of 0.2 m. Find its moment of inertia about a diameter.

formula

Q4. A solid sphere has a mass of 5 kg and a radius of 0.3 m. Find its moment of inertia about a tangent.

formula

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