# Inertia and Wr^2 GD^2 and mr^2

1. Mar 1, 2005

### knarl

trying to figure out units for inertia.

am I right in saying some units for inertia are?:

imperial units - not interested (Wr^2)
kgfm^2 (GD^2)
kgm^2 (mr^2)

if so, what is the f and the G and the D?

Thanks for any help.

2. Mar 1, 2005

### Andrew Mason

The units are that of mass: kg, g, lb, oz. Mass is a measure of the inertia of an object - the ratio of force to acceleration. Inertia = f/a = m

AM

3. Mar 1, 2005

### knarl

sorry, I meant moments of intertia. ie for a rotating body. It's just really the f that is putting me off.

4. Mar 1, 2005

### dextercioby

That's an ancient (b4 1961) unit for force.It's Kgf=Kilogram-force...The gravitational acceleration (average) at the surface of the Earth times 1Kg.That explains G...As for D,i guess it's probably distance (?)...

The last of the 3 units presented is the correct (SI-mKgs) one.

Daniel.

5. Mar 1, 2005

### Andrew Mason

Ok. Just use $$I = \tau/\alpha$$.

Since torque is in units of force * distance or Nm, and $\alpha$ is $rad/sec^{2}$, the units are $Nmsec^2$ (or $kgm^2$ since $N = kgm/sec^2$). As dexter says, kgf is kilograms force which is 1 kg x 9.8m/sec^2.

AM

6. Mar 1, 2005

### knarl

thanks guys, I had a hunch that was what the f was. And the D is diameter.

knarl.

7. Mar 1, 2005

### dextercioby

Weight times diameter doesn't make any sense...Even elimination would lead you to the correct answer.

Daniel.