9.7 : Theorems of Pappus and Guldinus

The two theorems developed by Pappus and Guldinus are widely used in mathematics, engineering, and physics to find the surface area and volume of any body of revolution. This is done by revolving a plane curve around an axis that does not intersect the curve to find its surface area or revolving a plane area around a non-intersecting axis to calculate its volume.

For finding the surface area, consider a differential line element that generates a ring with surface area dA when revolved. Integrating this differential area determines the surface area of the revolution, which is described by the first theorem. The first theorem states that the surface area of revolution is equal to the product of the length of the generating curve and the distance traveled by its centroid during the generation of the surface area.

Similarly, for calculating the volume of revolution, consider a differential area element generating a ring with differential volume when revolved. Integrating this differential volume determines the volume of revolution, as per the second theorem. The second theorem of Pappus and Guldinus states that the volume of revolution is equal to the product of the generating area and the distance traveled by its centroid in generating such volume. Understandably, if either curve or area is revolved through an angle instead of a complete 360° rotation, their respective formulae would need to be altered accordingly.