# Moment of inertia of a point mass

Hello,

This is a very basic question. Wikipedia says moment of inertia I of a point mass m is calculated as:

I = m * r^2

Where, r is distance from the axis of rotation. What this actually mean? I mean what is an r?

## Answers and Replies

DrClaude
Mentor
Where, r is distance from the axis of rotation. What this actually mean? I mean what is an r?

It is a distance, expressed in units of length (meters in the SI).

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this is axis of rotation and in front ist point mass not full stop .

it's at r dist from rod

I=mr2

moment of inertia of a point mass with respect to an axis is the product of mass times the distance of the axis
squared.So "r" is the distance of the mass "m" from the axis of roation.

sophiecentaur
Science Advisor
Gold Member
2020 Award
Moment of Inertia of an object has to be referenced to a particular point or axis. Take any object and its MI is a minimum when that reference is through the CM. For a point mass, it is Zero.