Understanding the Application of the Parallel-Axis Theorem

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In summary, the conversation discusses the calculation of moment of inertia for a rotating solid sphere. It is determined that the moment of inertia is equal to (2/5)MR^2 + M*D^2, where D is the distance from the center of mass to the desired axis of rotation, whether it is inside or outside the body.
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Niles
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Homework Statement


If I have a solid sphere (I = (2/5)MR^2) and it rotates around a point on an axis that is 5m away (it has radius 0.1m) - is the moment of inertia then: I = (2/5)MR^2 + M*(4.9m)^2?

My question is - does the parallel-axis only work when we are moving the axis of rotation around "inside" the body or also outside it?
 
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It works inside or outside. I'd say that I = (2/5)MR^2 + M*D^2, where D is the distance from the center of mass to the desired axis.
 
  • #3
Of course, my bad - but thanks again. You have helped me very much.
 

1. What is the Parallel-axis theorem?

The Parallel-axis theorem is a mathematical principle that relates the moment of inertia of an object about its center of mass to the moment of inertia about a parallel axis. It states that the moment of inertia about a parallel axis is equal to the moment of inertia about the center of mass plus the product of the mass of the object and the square of the distance between the two axes.

2. How is the Parallel-axis theorem used in physics?

The Parallel-axis theorem is used in physics to calculate the moment of inertia of a rigid body about a given axis. It allows for the simplification of complex systems and the prediction of rotational motion. It is also used in engineering and design to determine the stability and strength of structures.

3. What is the difference between the Parallel-axis theorem and the Perpendicular-axis theorem?

The Parallel-axis theorem and the Perpendicular-axis theorem are two different principles that relate to the moment of inertia of a rigid body. The Parallel-axis theorem applies when the axis of rotation is parallel to the axis passing through the center of mass, while the Perpendicular-axis theorem applies when the axis of rotation is perpendicular to the axis passing through the center of mass.

4. Can the Parallel-axis theorem be applied to any shape or object?

Yes, the Parallel-axis theorem can be applied to any shape or object as long as it is rigid. It is a general principle that applies to all types of bodies, including point masses, rods, disks, and irregular shapes. The only requirement is that the object has a well-defined moment of inertia about its center of mass.

5. How is the Parallel-axis theorem derived?

The Parallel-axis theorem is derived using mathematical calculations and the principles of rotational dynamics. The proof involves the use of the parallel axis theorem for a point mass, the definition of the moment of inertia, and the application of the parallel axis theorem to the infinitesimal elements of a rigid body. The complete proof can be found in most physics or engineering textbooks.

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