A system in which the (vector) momentum of a particle is conserved in the absence of external forces. Thus, only in an inertial system can Newton's laws of motion be appropriately applied. For all purposes in meteorology, a system with origin on the axis of the earth and fixed with respect to the stars (absolute coordinate system) can be considered an inertial system. When relative coordinate systems are used, moving with respect to the inertial system, apparent forces arise in Newton's laws, such as the Coriolis force.