Could someone help me understand why the density of a homogeneous disk is $\dfrac{m}{(\pi a)^2}$?
I am trying to understand an example about finding the moment of inertia of an object.
The question is : Find the moments of inertia with respect to the -axis, the -axis, and the origin of a thin homogeneous disk of mass and radius , centered at the origin.
And the example starts with this: Since the disk is homogeneous, its density is constant and given by $$r(x, y)=\frac{m}{(\pi a)^2}$$.
Well, I really don't understand why the density is $\frac{m}{(\pi a)^2}$. Coulde someone help me with this?