be called: the Coriolis effect; The gravitational force would also be a fictitious force (pseudo force), based upon a field model in which particles distort spacetime due to their mass, such as in the theory of general relativity. Assuming Newton's second law in the form F = ma, fictitious forces are always proportional to the mass m. The fictitious force that has been called an inertial force is also referred to as a d'Alembert force, or sometimes as a pseudo force. D'Alembert's principle is just another way of formulating Newton's second law of motion. It defines an inertial force as the negative of the product of mass times acceleration, just for the sake of easier calculations. (A d'Alembert force is not to be confused with a contact force arising from the physical interaction between two objects, which is the subject of Newton's third law – 'action is reaction'. In terms of the example of the passenger vehicle above, a contact force emerges when the body of the passenger touches the backrest of the seat in the car. It is present for as long as the car is accelerated.) Four fictitious forces have been defined for frames accelerated in commonly occurring ways: one caused by any acceleration relative to the origin in a straight line (rectilinear acceleration); two involving rotation: centrifugal force and Coriolis effect and a fourth, called the Euler force, caused by a variable rate of rotation, should that occur. Background The role of fictitious forces in Newtonian mechanics is described by Tonnelat: Fictitious forces arise in classical mechanics and special relativity in all non-inertial frames. Inertial frames are privileged over non-inertial frames because they do not have physics whose causes are outside of the system, while non-inertial frames do. Fictitious forces, or physics whose cause is outside of the system, are no longer necessary in general relativity, since these physics are explained with the geodesics of spacetime: "The field of all possible space-time null geodesics or photon paths unifies the absolute local non-rotation standard throughout space-time.". On Earth The surface of the Earth is a rotating reference frame. To solve classical mechanics problems exactly in an Earthbound reference frame, three fictitious forces must be introduced: the Coriolis force, the centrifugal force (described below) and the Euler force. The Euler force is typically ignored because the variations in the angular velocity of the rotating surface of the Earth are