Afictitious force, also called apseudo force, d'Alembert force orinertial force,is an apparentforcethat acts on all masses whose motion is described using anon-inertial frame of reference, such as arotating reference frame.

The forceFdoes not arise from anyphysical interactionbetween two objects, but rather from the accelerationaof thenon-inertial reference frameitself. As stated by Iro:

Such an additional force due to nonuniform relative motion of two reference frames is called apseudo-force.

—H. Iro inA Modern Approach to Classical Mechanicsp. 180

AssumingNewton's second lawin the formF=ma, fictitious forces are always proportional to the massm.

A fictitious force on an object arises when the frame of reference used to describe the object's motion is accelerating compared to a non-accelerating frame. As a frame can accelerate in any arbitrary way, so can fictitious forces be as arbitrary (but only in direct response to the acceleration of the frame). However, four fictitious forces are defined for frames accelerated in commonly occurring ways: one caused by any relative acceleration of the origin in a straight line (rectilinearacceleration);two involving rotation:centrifugal forceandCoriolis force; and a fourth, called theEuler force, caused by a variable rate of rotation, should that occur.Gravitational forcewould also be a fictitious force based upon a field model in which particles distortspacetimedue to their mass.