# What are the units for inertia?

1. May 11, 2012

Hello there, there have been a lot of conflicts between my physics teachers and university lecturers about the the units for intertia. Some say there aren't any units for inertia, and others say that it's in n/s. However, I don't know who to believe anymore... thats why I'm here

Also, if there aren't any units for inertia, why is that? Also, does "moment of inertia" differ from "inertia"? If so, how and why?

2. May 11, 2012

### rollcast

The unit for inertia is the same as the units for mass, kg, the unit of moment of inertia is $$kg.m^{2}$$

Inertia is the resistance to linear acceleration by a force applied to the body.

Moment of inertia is the resistance to angular acceleration by torque applied to the body.

Last edited: May 11, 2012
3. May 12, 2012

### xAxis

Or as they say: "Mass is a measure of inertia"

4. May 12, 2012

### Fewmet

Miniradman: I find it surprising your professors find the point debatable. I think rollcast's answer is pretty unambiguous.

If you find it easier to start with a slightly less mathematical definition, mass determines how hard it is change a body's translational motion. Moment of inertia is determines how hard it is to change a body's rotational motion.

5. May 12, 2012

### D H

Staff Emeritus
rollcast's answer may be unambiguous, but the term "inertia" is not. Does it mean mass or momentum? That's why you won't see the term used much in physics texts. Why use an ambiguous term when there are perfectly good, unambiguous alternatives (i.e., mass and momentum)?

6. May 14, 2012

### sophiecentaur

Hear hear.

7. Jul 14, 2013

### honglin_8

inertia is unambiguous

First there are two terms: Inertia vs. Moment of Inertia
Second, they have different units: kg vs. kg*m2 as rollcast posted.
Third, they are different from mass or anything else:
Inertia - The tendency of a body to resist acceleration; the tendency of a body at rest to remain at rest or of a body in straight line motion to stay in motion in a straight line unless acted on by an outside force
Mass - a collection of incoherent particles, parts, or objects regarded as forming one body
Fourth, they are not mass or momentum, it has its own property. Even though the Inertia has the same unit as mass, the concept is different. The Moment of Inertia does not even have the same unit as momentum.