When to use summation vs integration for moment of inertia?

[Neon32]

To find Moment of inertia, there are 2 rules:
I = ∑m r²
I= ∫ r² dm

I don't know when to use the first and when to use the second rule.

 

[Doc Al]

I don't know when to use the first and when to use the second rule.

The first is useful for point masses; the second, for continuous distributions of mass.

 

[Neon32]

The first is useful for point masses; the second, for continuous distributions of mass.

what does continuous distributions of mass mean?

 

[Doc Al]

what does continuous distributions of mass mean?

Check this out: Moment of Inertia Examples