Perhaps the most-common gravitational calculation is to determine the "weight" of an object of mass mo sitting on the surface of the earth (i. e., the magnitude of the gravitational attraction between the earth and the object). According to Newton's law of universal gravitation, the gravitational force on the object is Fg=G(memor2e)=(Gmer2e)mo, where me is the mass of the earth and re is the distance between the object and the center of the earth (i. e., re = radius of the earth = 6.38×103km).