The point at which the total area of a plane figure or lamina is assumed to be concentrated is called centroid
Centroid of a Line
Centroid of an Area
Characteristics of Centroid
Centroid Of Regular Figures
Centre of Gravity
Centre of gravity is the point about which the resultant of the whole weight of the body may be considered to act. It is denoted by G
A. CENTROID OF COMPOSITE FIGURES
Note : If the area has a hole or cut out portion, the first moment of inertia and area must be subtracted to yield the centroid
B. PAPPUS GULDINUS THEOREMS
Pappus Guldinus Theorems are two theorems describing a simple way to calculate volumes (solids) and surface areas (shells) of revolution.
The surface area A of a surface of revolution generated by rotating a plane curve about an axis external to it and on the same plane is equal to the product of the arc length of the curve and the distance y traveled by its geometric centroid.
The second theorem states that the volume V of a solid of revolution generated by rotating a plane area about an external axis is equal to the product of the area A and the distance y traveled by its geometric centroid