The gravitational field experienced by a particle of a certain mass due to another particle is the gravitational force acting on that particle divided by its mass. The principle of superposition states that the net gravitational field at any point is the vector sum of the gravitational fields due to individual point objects. A real object with a definite shape can be divided into infinitesimal parts, each having a differential mass producing a differential field at a certain distance. The resultant field due to the entire object is obtained according to the superposition principle by integrating over a suitable limit. For example, consider a sphere with constant density. The differential mass for any constituent shell can be written as density times the differential volume. The field equation for a shell is known, into which the differential mass is substituted. The expression is then integrated from zero to the radius of the sphere. Since density times volume gives the mass, the final field obtained is the same as that of a point mass.