Radius of Gyration: Solving Inertia Discrepancies

In summary, the conversation discusses the calculation of the inertia of a disk and why it cannot be determined using the formula for a rotating disk. The reason is that the spool is not a uniform disk and the effective radius must be used instead. The conversation ends with an acceptance of the given effective radius.
  • #1
theBEAST
364
0

Homework Statement


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The Attempt at a Solution


Alright, so the solution manual shows that the intertia of the disk is:
I = m * k2, where k is the radius of gyration​

However why can't the inertia of the disk be:
I = 0.5 * m * r2, where r is the radius of the disk​
This is the formula for the inertia of a rotating disk... However when I plug in all my numbers I get a different inertia value when compared to the inertia value that I got using the radius of gyration.
 
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  • #2
Because the spool is not a uniform disk. You don't need to know why it's not uniform, just accept that you are given the effective radius.
 
  • #3
haruspex said:
Because the spool is not a uniform disk. You don't need to know why it's not uniform, just accept that you are given the effective radius.

Ah okay, I wish it said that the density is not uniformly distributed... But thanks!
 

Related to Radius of Gyration: Solving Inertia Discrepancies

What is the radius of gyration?

The radius of gyration, also known as the gyradius, is a measure of the distribution of mass around an axis. It is defined as the distance from the axis at which the entire mass of an object would have to be concentrated to give the same moment of inertia as the object has with its actual distribution of mass.

Why is the radius of gyration important?

The radius of gyration is important because it helps us understand an object's resistance to rotation. It is a key factor in calculating an object's moment of inertia, which is a measure of an object's resistance to changes in rotational motion. It is also used in many engineering and physics calculations related to rotational motion.

How is the radius of gyration calculated?

The radius of gyration is calculated by taking the square root of the object's moment of inertia divided by its mass. This can be represented by the formula k = √(I/m), where k is the radius of gyration, I is the moment of inertia, and m is the mass of the object.

What is the difference between the radius of gyration and the moment of inertia?

The radius of gyration is a measure of the distribution of mass around an axis, while the moment of inertia is a measure of an object's resistance to changes in rotational motion. The radius of gyration is used to calculate the moment of inertia, but they are not interchangeable terms.

How can the radius of gyration help solve inertia discrepancies?

The radius of gyration can help solve inertia discrepancies by providing a more accurate representation of an object's moment of inertia. In some cases, using the radius of gyration instead of the actual moment of inertia can simplify calculations and provide more accurate results. This can be especially useful when dealing with complex objects with irregular mass distributions.

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