Peoples Physics Concepts

9.2. Moment of Inertia http://www.ck12.org

9.2 Moment of Inertia

• Define and calculate momentum of inertia.

Students will learn what is momentum of inertia, what determines a large or small moment of inertia for an object

and how to calculate moment of inertia.

Key Equations

I=∑miri^2

Guidance

Moment of inertia tells you how difficult it is to rotate an object. It is equivalent to the mass in linear problems.

Moment of inertia is proportional to an objects mass and to it’s distance from the rotational axis squared. The

distance from the rotational axis dominates over the objects mass due to the square power. Thus the more mass an

object has at it’s ’edges’ the more moment of inertia it has. Thus if you grab a big long pole and hold it at the center,

it is fairly easy to rotate. However, if you hold the same pole at the end and try to rotate it -not so easy! Finally,

moment of inertias can be added as long as the rotating parts in question are rotating around the same axis. For

example, the moment of inertia of a pole with a mass at its end is the sum of the individual moment of inertias of

each object involved. See example 2 for another problem where the moment of inertias are added together.

Example 1