units are not compatible

Pretty simple to see.

You can't add apples to pears.

M.Omax has unit N,

M.k1 has unit kN*mm

Those units are not the same, so you cannot add them, even if you squared the variables.

Success!
Luc

your first 3 moments are in N because 1/2 has no unit (it is a lenght?). Add a unit to 1/2 !