Correct Use of the Parallel Axis Theorem for Moment of Inertia

In summary, the Parallel Axis Theorem is a principle used in physics and engineering to determine the moment of inertia of a rigid body about any axis, given its moment of inertia about a parallel axis through its center of mass. The theorem states that the moment of inertia (I) about the new axis is equal to the moment of inertia about the center of mass axis (I_cm) plus the product of the mass (m) of the body and the square of the distance (d) between the two axes: I = I_cm + md². Correct application involves ensuring that the axes are parallel and accurately measuring the distance between them. This theorem simplifies calculations for complex shapes by allowing the use of known moments of inertia and is widely utilized in mechanical
  • #1
simphys
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Homework Statement
In order to increase her moment of inertia about a vertical axis, a spinning figure skater stretches out her arms horizontally; in order to reduce her
moment of inertia, she brings her arms down vertically along her sides. Calculate the change of moment of inertia between these two configurations of the arms. Assume that each arm is a thin, uniform rod of length 0.60 m and mass
2.8 kg hinged at the shoulder at a distance of 0.20 m from the axis of rotation.
Relevant Equations
xD
so calculated, the moment of inertia for a rod about an axis at the end of the rod is I = 1/3 * M * L^2
here for case 1: arms to the side
I is calculated to be ##I = 0.224##

for case 2: arms stretched
## I = 1 / 3 * M * L^2 + M * d^2 ## with L = 0.6 m (length of rod) and d = 0.2 (dinstance from 'hinge' to axis of rotation)
## I = 0.896## is my answer.
Yet the actual answer is ##I = 1.568##. I don't really understand what I am doing wrong here so can someone help me please?

Reasoning:
In case 2 I basically have the moment of inertia at the end (the 'hinge'/shoulder so to say) and need to add a dinstance d to get the moment of inertia about the rotation axis via the parallel axis theorem yet this isn't correct.

Thanks in advance
 
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  • #2
simphys said:
for case 2: arms stretched
## I = 1 / 3 * M * L^2 + M * d^2 ## with L = 0.6 m (length of rod) and d = 0.2 (dinstance from 'hinge' to axis of rotation)
## I = 0.896## is my answer.
You have not used the parallel axis theorem correctly. $$I = I_{\rm cm} + md^2$$ What does ##I_{\rm cm}## mean?
What does ##d## mean?
 
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  • #3
TSny said:
You have not used the parallel axis theorem correctly. $$I = I_{\rm cm} + md^2$$ What does ##I_{\rm cm}## mean?
What does ##d## mean?
yeah apologies it should have been 2 times the symbolic equation, just forgot to type it out.
Omg... totally forgot that... thanks a lot... I needed to use the moment of inertia about the center of mass basically..,.
 
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FAQ: Correct Use of the Parallel Axis Theorem for Moment of Inertia

What is the Parallel Axis Theorem?

The Parallel Axis Theorem is a fundamental principle in mechanics that allows you to determine the moment of inertia of a body about any axis, given the moment of inertia of the body about a parallel axis through its center of mass and the distance between the two axes. The theorem states that I = I_cm + Md², where I is the moment of inertia about the new axis, I_cm is the moment of inertia about the center of mass axis, M is the mass of the body, and d is the distance between the two parallel axes.

When should the Parallel Axis Theorem be used?

The Parallel Axis Theorem should be used when you need to calculate the moment of inertia of an object about an axis that is parallel to and a certain distance away from the axis through its center of mass. This situation commonly arises in engineering and physics problems where the rotation axis does not pass through the center of mass.

How do you apply the Parallel Axis Theorem to composite shapes?

To apply the Parallel Axis Theorem to composite shapes, you need to break the composite shape into simpler components whose moments of inertia about their own center of mass axes are known or easier to calculate. Then, use the Parallel Axis Theorem to shift each component's moment of inertia to the desired axis. Finally, sum the moments of inertia of all components to get the total moment of inertia about the desired axis.

What are common mistakes to avoid when using the Parallel Axis Theorem?

Common mistakes include: not correctly identifying the center of mass of the object, incorrectly calculating the distance between the parallel axes (d), forgetting to square the distance (d²), and not including the mass (M) of the object in the calculation. It's also important to ensure that the axis you are shifting to is indeed parallel to the center of mass axis.

Can the Parallel Axis Theorem be used for non-rigid bodies?

The Parallel Axis Theorem is primarily applicable to rigid bodies. For non-rigid bodies, the distribution of mass can change with deformation, which would affect the moment of inertia. Therefore, the theorem is not generally used for non-rigid bodies as it assumes a fixed mass distribution.

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