Let a planer body occupy a region and have density (mass per unit area) to be at a point in the body. Then the mass of the body is given by

Further the point , where

and

is called the center of mass of the body. This is the point where the whole mass of the body can be assumed to be concentrated. For , if is the density (mass per unit volume) at a point , then the mass of is defined by

and the center of mass is given by