# Body inertness

1. Apr 29, 2015

### bagofbones

I have a question about body inertness. Is moment of inertia is unambiguous inertness characteristic?

2. Apr 29, 2015

### Staff: Mentor

Not sure what you mean by "inertness". (Do you mean inertia?) In any case, moment of inertia has a clearly defined meaning.

3. Apr 29, 2015

### brainpushups

If I understand your question correctly - the moment of inertia is 'ambiguous' in the sense that it is ill-defined if you have not specified the axis of rotation. For example, the moment of inertia of a disc about its center of mass is 1/2 M R2 which is different than its moment about a point on its circumference (3/2 M R2)

4. Apr 29, 2015

### rumborak

If I understand the question correctly, the question is whether the moment of inertia is intrinsic to a body.
The answer would be, it is intrinsic to the specific *shape* and mass distribution of the body. If you change the shape, or add/remove mass, the moment of inertia changes.
EDIT: And yes, as brainpushups points out, also dependent on the axis of rotation.

5. Apr 29, 2015

### bagofbones

Thanks. One more if you don't mind. Why, when you use rotary pendulum (I don't know official term), it is device like this:

Where you put body in it and measure Periods, while it rotates, then calculate inert moment I. So, why it is recommended to use small angle of the frame while measuring periods? For better and more accurate results?

6. Apr 29, 2015

### rumborak

I think it's more of a practical reason. You need to ensure that the thread/wire the body is suspended on, which twists when the body is rotating, needs to have a steady counter-torque. If you twist it too much you can't guarantee the linearity of it anymore.

7. Apr 29, 2015

### sophiecentaur

Also I think that the friction of the air could be relevant for high speeds.