- #1
bagofbones
- 2
- 0
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?
- Thread starter bagofbones
- Start date
In summary, the conversation discusses the concept of moment of inertia and its ambiguity when the axis of rotation is not specified. It is noted that the moment of inertia is intrinsic to the shape and mass distribution of a body and can change if these factors are altered. The conversation then moves on to discussing the use of a rotary pendulum device for measuring the moment of inertia, and why it is recommended to use a small angle of rotation for more accurate results. This is because a larger angle may affect the linearity of the suspension wire and introduce air friction at high speeds.
- #2
Doc Al
Mentor
- 45,586
- 2,306
Not sure what you mean by "inertness". (Do you mean inertia?) In any case, moment of inertia has a clearly defined meaning.
- #3
brainpushups
- 453
- 197
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
Gold Member
- 29,515
- 7,145
Also I think that the friction of the air could be relevant for high speeds.rumborak said:
FAQ: Moment of Inertia: Is it an Unambiguous Body Inertness Characteristic?
What is moment of inertia?
Moment of inertia is a physical quantity that measures an object's resistance to changes in its rotational motion. It is a measure of an object's mass distribution around its axis of rotation.
How is moment of inertia calculated?
Moment of inertia is calculated by multiplying the mass of each particle of an object by the square of its distance from the axis of rotation and then summing up all these values.
Is moment of inertia the same as mass?
No, moment of inertia and mass are two different physical quantities. Mass measures an object's resistance to changes in linear motion, while moment of inertia measures its resistance to changes in rotational motion.
Why is moment of inertia an important characteristic of an object?
Moment of inertia is important because it determines how an object will behave when subjected to rotational motion. It is crucial in understanding and predicting an object's rotational dynamics, such as its angular acceleration and stability.
Can the moment of inertia of an object change?
Yes, the moment of inertia of an object can change if its mass distribution changes. For example, if an object's mass is redistributed further away from its axis of rotation, its moment of inertia will increase, making it more resistant to changes in rotational motion.