Question

Hello, I have a doubt about inertia in a uniform bar and axis of rotation..
What's the difference between a lengthwise axis and a parallel one?
In some books I have found that when we're talking about lengthwise axis that pass through the center of the bar the inertia of the bar is equal to the inertia of a cylinder, and when it's about parallel axis that pass through the bar , the inertia is zero. I'm really confused with that words...
I'm not talking about the parallel axis theorem...
Thanks for any answer

Answer 1

https://www.physicsforums.com/threads/moment-of-inertia-of-a-rod-at-an-angle.449943/

Answer 2

If it passes through the bar along the axis of the bar, the moment of inertia is small because the mass is close to the axis of rotation. Remember that mR^2 stuff.
It is not zero though unless the radius of the bar is zero :) It is in fact a slender solid cylinder and its moment of inertia is
(1/2) mR^2

in the other direction, like spinning a baton about the center, it is much bigger
(1/12) m L^2
Note - L^2 is presumably HUGE compared to R^2