The discussion revolves around determining the moment of inertia for a composite object, specifically regarding the appropriate calculations when summing moments about the object's centroid. The subject area includes concepts of rotational dynamics and moment of inertia for various shapes.
The discussion is ongoing, with participants raising questions about the assumptions made regarding the shapes and their properties. Some guidance is offered regarding the use of the parallel axis theorem and the need for clarity on the axis of rotation. There is recognition of uncertainty in the original poster's assumptions, indicating that more information may be necessary to proceed effectively.
Participants note that the problem may lack sufficient detail, leading to multiple interpretations of the shapes and their respective moments of inertia. The original poster's guesses about the shapes and their properties are acknowledged, but there is a consensus that assumptions need to be supported with reasoning.
Simon Bridge said:
... as opposed to, say, a solid sphere... or a solid disk... what are you basing your guess on?
... why a hollow for the first shape and solid for the second one? Why not the other way around or maybe both hollow or both solid?
... OK. The working you did is for rotating about an axis along the length of the "cylinder". For an axis out of the page, where is the axis of rotation on the figure? The correct moment of inertia depends on it's position. Or, maybe, the question is just asking for which com inertias to apply the parallel axis theorem to? In which case, you want to look more closely at which com moment you have used for the "cylinder".
