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Moment Of Inertia Of A Hollow Sphere

Moment Of Inertia Of A Hollow Sphere

In rotational motion, the Moment of Inertia (MI) measures resistance to changes. It depends on the total mass of the body and the distribution of mass relative to the axis of rotation. A hollow sphere is a spherical shell with mass concentrated on its surface and a negligible thickness.

Moment of Inertia Of A Hollow Sphere about its Diameter

formula

Derivation:

Surface mass density:

The total surface area of a sphere = 4πR2

Surface mass density,

formula

Using spherical coordinates:

In spherical coordinates, a ring at polar angle θ has radius R sin θ and circumference 2πR sin θ

The width of the ring = R dθ

Area of the ring = 2πR sin θ × R dθ = 2πR2 sin θ dθ

formula

Distance from the rotation axis:

For rotation about the z-axis (diameter), each point on the ring is at a distance R sin θ from the axis

Elemental moment of inertia:

formula

Integration:

formula

This formula is only valid for a thin spherical shell.

Applications

  • Astronomy: Modelling planets with dense cores and lighter outer shells.
  • Sports: Designing balls like footballs, cricket balls, etc, with specific rolling and spinning properties.
  • Engineering: Flywheels or rotating shells in machines.
  • Defence: Shell-like casings in projectiles and space equipment.

Summing Up

formula

Frequently Asked Questions

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

Moment of inertia increases if mass is farther from the axis; for this reason, the hollow sphere has a higher moment of inertia for the same mass and radius.

Q2. Why does a solid sphere roll faster than a hollow sphere?

A solid sphere rolls faster than a hollow sphere on an inclined plane (assuming no slipping), because more energy goes into rotational motion in the hollow sphere.

Q3. How does thickness affect the moment of inertia?

If the sphere is thick, integration must consider the volume distribution, which is why the given formula applies only for negligible thickness.

Q4. A hollow sphere has a mass of 5 kg and a radius of 0.4 m. Find its moment of inertia about a diameter.

formula

Q5. A hollow sphere of radius 0.6 m has a moment of inertia of 0.96 kg·m2 about its diameter. Find its mass.

formula

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