# Is this a correct derivation of Moment of Inertia?

1. Jul 21, 2013

### Marmoteer

Hello everybody!
This is the derivation (for a single particle).
$$\tau = F_{\perp }r \ = ma_{\perp}r \ = \alpha mr^2 \\ \text{if }\ \tau = I\alpha \text{ where } I \text{ is resistance to accleration then } \\ I = mr^2$$
I'm curious what the problem with this is because I haven't seen it in any of my physics texts (serway and university at least). I also haven't seen it on the interwebs very much.

2. Jul 21, 2013

### Andrew Mason

Moment of inertia refers to the ratio of the torque to angular acceleration of a rigid body about an axis. It is the inertial resistance to rotation that a rigid body has about a specific axis of rotation. A single particle is not a rigid body.

What you are doing is determining the ratio of torque to angular acceleration of a single point particle about an axis of rotation.

If you were to reduce a rigid body to a collection of single point particles of mass dm and add up all the dmr2 terms where r is the distance from the point particle to the axis of rotation, you would end up with the moment of inertia of the rigid body.

AM