When a force is only operative for a very short interval of time, and during this time any changes which occur in them cannot be traced; it is only the total change after collision that can be observed. If a perpendicular is erected through the centres of the small surfaces, this perpendicular indicates the line of action of the forces. If the line passes through the centres of gravity of the bodies, the collision is said to be central, otherwise it is called eccentric. In spheres these perpendiculars always pass through the centres, which points are also their respective centres of gravity if the spheres are uniform in density. When impact occurs between a sphere and infinite wall, both of which are perfectly elastic, then the sphere is reflected from the wall with a velocity equal to that which it originally possessed but in such a direction that the angle of incidence is equal to the angle of reflexion.