What is the relationship between the moment of inertia and the length of a rod?

merlos
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The moment of inertia about an axis along the length of a rod is zero, correct?
 
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merlos said:
The moment of inertia about an axis along the length of a rod is zero, correct?
Almost. If the rod has a measurable radius, it is a cylinder.
 
In all the other parts ot the problem though I considered it a rod.

Here's the problem:

Find the moment of inertia about each of the following axes for a rod that is 0.280 cm in diameter and 1.70 m long, with a mass of 5.00×10−2 kg.

A. About an axis perpendicular to the rod and passing through its center.
I = (1/12)ML^2
I = .012 kgm^2

B. About an axis perpendicular to the rod passing through one end.
I = (1/3)ML^2
I = .048 kgm^2

C. About an axis along the length of the rod.
 
merlos said:
In all the other parts ot the problem though I considered it a rod.

Here's the problem:

Find the moment of inertia about each of the following axes for a rod that is 0.280 cm in diameter and 1.70 m long, with a mass of 5.00×10−2 kg.

A. About an axis perpendicular to the rod and passing through its center.
I = (1/12)ML^2
I = .012 kgm^2

B. About an axis perpendicular to the rod passing through one end.
I = (1/3)ML^2
I = .048 kgm^2

C. About an axis along the length of the rod.
The rod has a diameter. It has a moment of inertia about the long axis.
 
So, I = (1/4)MR^2 + (1/3)ML^2 ?
 
The length of the stick--or cylinder--parallel to the axis doesn't matter, only the radius.
 

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