Consider a surface of density (mass per unit area) . Then the mass of can be defined to be
The moments of about the three axes planes is defined by
Further the point is called the center of mass of , where
of density (mass per unit area) 
. Then the mass of 
can be defined to be 
about the three axes planes is defined by 
is called the center of mass of 
, where 
represent the velocity field of a fluid flow in space at a point 
. Let 
be its density at 
. Then 
