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Moment Of Inertia Of A Ring

Moment Of Inertia Of A Ring

The moment of inertia (MI) measures an object’s resistance to rotational motion about a specific axis. It determines resistance to angular acceleration and depends not only on the mass of the object but also on the distribution of that mass relative to the axis of rotation. A ring (or circular hoop) is a thin circular object with all its mass concentrated at a fixed radius from the centre.

Moment Of Inertia Of A Ring about Different Axes

Consider a thin ring of mass M, radius R, and negligible thickness.

Moment of Inertia About the Central Axis (Perpendicular to the Plane)

Considering the ring rotates about an axis perpendicular to its plane and passing through the centre (central axis), the mass element of the ring is at the same distance R from the axis.

By the definition of moment of inertia:

I = ∫ r² dm

Since r = R for all mass elements dm, the integral simplifies to:

Icentral = ∫ R² dm

= R² ∫ dm

= R² M

Icentral = MR²

This shows that the moment of inertia of a thin ring is directly proportional to its mass and the square of its radius.

formula

Moment of Inertia About a Diameter (In-Plane Axis)

For rotation about an axis lying in the plane of the ring and passing through the centre, the perpendicular axis theorem is applied.

formula

Moment of Inertia About Any Parallel Axis

If the axis of rotation is parallel to the central axis but does not pass through the centre of mass, the parallel axis theorem is used:

I = ICM + Md²

Where:

ICM – moment of inertia about the central axis through the centre of mass

d – distance between the axes

Formulae at a Glance

Shape Axis Formula
Thin Ring Central axis (perpendicular to the plane) I = MR²
Thin Ring Diameter (in-plane axis) formula
Thin Ring Parallel axis I = ICM + Md²

Applications

  • Flywheels – Store rotational energy efficiently.
  • Vehicle wheels – Transfer torque smoothly.
  • Pulleys – Facilitate lifting and mechanical power transmission.
  • Gears – Transmit rotational motion between components.
  • Mechanical bearings – Reduce friction in rotating machinery.
  • Hoops in sports equipment – Allow uniform rotation in apparatus like hula hoops.

Summing Up

formula

Frequently Asked Questions

Q1. Why is the moment of inertia of a ring larger than a disc?

The moment of inertia of a ring is larger because all the mass of the ring lies at the maximum distance from the axis, increasing resistance to rotation.

Q2. A thin ring of mass 3 kg and radius 0.4 m rotates about its central axis. Find its moment of inertia.

Given: Mass = 3 kg, Radius = 0.4 m

I = MR²

= 3 × (0.4)²

= 3 × 0.16

= 0.48 kg·m²

Q3. A thin ring has a mass of 2 kg and a radius of 0.5 m. Find its moment of inertia about the central axis perpendicular to its plane.

Given: Mass = 2 kg, Radius = 0.5 m

I = MR²

= 2 × (0.5)²

= 0.5 kg·m²

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