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3D lab · Rotational motion

Moment of Inertia About Different Axes (3D)

Spin a rod, ring, disc, sphere or plate about different 3D axes and read I, the radius of gyration and rotational kinetic energy.

Last updated: 26 October 2026

Direct answer

Spin a rod, ring, disc, sphere or plate about different 3D axes and read I, the radius of gyration and rotational kinetic energy.

The same body is harder to spin about an axis far from most of its mass. Compare the central axis with the parallel axis at the edge: the difference is exactly Md².

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Moment of Inertia About Different Axes (3D)

Rotational motion

Drag to rotate, two fingers or right-drag to pan, scroll or pinch to zoom. The toolbar has views, cutaway, measure and “show the maths”.

What's happening? The same body is harder to spin about an axis far from most of its mass. Compare the central axis with the parallel axis at the edge: the difference is exactly Md².
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Terms used on this page

Moment of inertia (I)Radius of gyration (k)

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FAQs

Frequently asked questions

What is the moment of inertia of a disc about a diameter?

MR²/4. By the perpendicular axis theorem it is half of MR²/2.

What is the parallel axis theorem?

I = I_cm + Md², where d is the distance between the parallel axes.

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