Inertial forces are not real, but apparent forces that are experienced by an observer in a non-inertial reference frame, reflecting the motion of the frame. Thus, you experience a backwards inertial force if you sit in an accelerating car, and a forward intertial force if the break is applied. If the reference frame is rotating, well known inertial forces are the centrifugal force and the Coriolis force. We also have the Euler force, which reflects the angular acceleration of the frame. But there is a fourth inertial force in a rotating 3-dimensional frame, which I don't know what it is called. This force reflects the change of the axis of rotation. If the frame has its origin fixed, this intertial force at a point with position vector r is Fi = - ω dn/dt × r, where ω is the angular velocity and n is the axis of rotation, a unit vector. For example, if the centre of the Earth is considered as fixed and the Earth's axis is changing so that the north pole is moving towards Greenwich, an observer at the north pole will experience an inertial force directed east, seen from Greenwich. Does anyone know what (if anything) this inertial force is called? Is it included in the Euler force (since it also deals with non-uniform rotation)? Also, does the inertial force which reflects linear acceleration (such as in the car example above) have a a name?