Correct Use of the Parallel Axis Theorem for Moment of Inertia

  • #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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1. What is the Parallel Axis Theorem for Moment of Inertia?

The Parallel Axis Theorem for Moment of Inertia is a formula used to calculate the moment of inertia of an object about an axis that is parallel to its center of mass. It states that the moment of inertia about the parallel axis is equal to the moment of inertia about the object's center of mass plus the product of the object's mass and the square of the distance between the two axes.

2. Why is the Parallel Axis Theorem important in physics?

The Parallel Axis Theorem is important in physics because it allows us to calculate the moment of inertia of an object about an axis that is not its center of mass. This is useful in situations where the object is rotating about a different axis or when we need to determine the rotational energy of an object.

3. How do you use the Parallel Axis Theorem to calculate moment of inertia?

To use the Parallel Axis Theorem, you first need to know the moment of inertia of the object about its center of mass. Then, you need to know the distance between the center of mass and the new axis of rotation. Finally, you can use the formula I = Icm + md2 to calculate the moment of inertia about the new axis, where Icm is the moment of inertia about the center of mass, m is the mass of the object, and d is the distance between the two axes.

4. Can the Parallel Axis Theorem be applied to any object?

Yes, the Parallel Axis Theorem can be applied to any object as long as its moment of inertia about its center of mass is known. This includes both simple and complex objects, as well as objects with irregular shapes.

5. What are some real-life applications of the Parallel Axis Theorem?

The Parallel Axis Theorem has many real-life applications, such as in the design of vehicles and machinery where the moment of inertia about different axes needs to be calculated. It is also used in sports, such as in figure skating and gymnastics, to determine the rotational energy of a person or object. Additionally, the Parallel Axis Theorem is important in engineering and physics research, where accurate calculations of moment of inertia are necessary.

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