Subject: Why is rotational inertia proportional to r squared instead of r?

I understand that particles of matter further from the axis of rotation in a 
rotating, rigid object resist any given angular acceleration more, since they 
experience a tangential acceleration that is proportional to their distance 
from the axis of rotation (r). But I do not understand why rotational inertia 
is found to be proportional to r squared, then, instead of just r.

Re: Why is rotational inertia proportional to r squared instead of r?