- #1
bagofbones
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I have a question about body inertness. Is moment of inertia is unambiguous inertness characteristic?
Moment of Inertia: Is it an Unambiguous Body Inertness Characteristic?
- Context: Undergrad
- Thread starter bagofbones
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Discussion Overview
The discussion revolves around the concept of moment of inertia and its characterization as a property of body inertness. Participants explore whether moment of inertia is an unambiguous characteristic of a body, considering factors such as axis of rotation and mass distribution.
Discussion Character
- Exploratory
- Technical explanation
- Conceptual clarification
- Debate/contested
Main Points Raised
- Some participants question the definition of "inertness," suggesting that "inertia" may be the intended term, while acknowledging that moment of inertia has a clear definition.
- It is noted that moment of inertia can be considered ambiguous if the axis of rotation is not specified, with examples provided for a disc's moment of inertia about different axes.
- Participants discuss whether moment of inertia is intrinsic to a body, with the view that it depends on the body's shape and mass distribution, and changes with alterations to these factors.
- A question is raised regarding the use of small angles in rotary pendulum measurements for calculating moment of inertia, with a suggestion that practical reasons related to counter-torque and linearity are involved.
- Concerns about air friction affecting measurements at high speeds are also mentioned as a relevant factor.
Areas of Agreement / Disagreement
Participants express differing views on the clarity and intrinsic nature of moment of inertia, indicating that multiple competing perspectives remain without a consensus on whether it is an unambiguous characteristic.
Contextual Notes
The discussion highlights the dependence of moment of inertia on the axis of rotation and the shape and mass distribution of the body, as well as practical considerations in measurement techniques that may affect results.
- #2
Doc Al
Mentor
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Not sure what you mean by "inertness". (Do you mean inertia?) In any case, moment of inertia has a clearly defined meaning.
- #3
brainpushups
- 457
- 195
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
rumborak
- 706
- 154
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.
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
bagofbones
- 2
- 0
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?
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
rumborak
- 706
- 154
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
sophiecentaur
Science Advisor
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Also I think that the friction of the air could be relevant for high speeds.rumborak said:
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