Mechanics studies the mutual actions and relative motion of material bodies either in solid, liquid or gaseous phase. Movement is studied in relation to a body, considered as a reference for movement. A reference system is considered fixed when it does not move in relation to another reference system, considered, conventionally, immobile. If the reference system moves relative to the fixed reference, then the reference system is said to be mobile. A reference system is called an inertial reference system if it has, with respect to a system considered fixed, a rectilinear and uniform movement. In mechanics, movement and rest have a relative character.