An acceleration of a parcel moving in a relative coordinate system. The total acceleration of the parcel, as measured in an inertial coordinate system, may be expressed as the sum of the acceleration within the relative system, the acceleration of the relative system itself, and the Coriolis acceleration. In the case of the earth moving with angular velocity Ω, a parcel moving relative to the earth with velocity u has the Coriolis acceleration 2Ω × u. If Newton's laws are to be applied in the relative system, the Coriolis acceleration and the acceleration of the relative system must be treated as forces. See apparent force, Coriolis force, inertial force, gravity.