Posted by chemstudent on .
The general formula for moment of inertia is:
I = Sigma mi * ri^2
(mi is mass of atom, ri is distance to axis of rotation)
In a diatomic molecule, the moment of inertia is
I = ma*mb(ma+mb) * R^2
(R is distance between atoms, ma and mb are masses of two atoms)
How is the diatomic molecule formula derived from the general formula? The two do not appear connected.

Chemistry 
GK,
You can find a proof in the web page below:
http://www.tutorvista.com/content/physics/physicsiii/rigidbody/diatomicmolecule.php
The symbols used are different but equivalent to the ones you are using. The general formula is not given explicitly but it is applied anyway. 
Chemistry 
chemstudent,
GK, thank you so much. That was exactly what I was looking for!