inertial forces

In classical mechanics or special relativity: Whenever an observer who is not an inertial observer wants to explain the movements of bodies using the law “force equals mass times acceleration”, that observer has to assume the existence of additional forces; these are called inertial forces. For ordinary forces like the electric force, the magnetic or the gravitational force, one can always state which bodies are acting on which other bodies; inertial forces, in contrast, appear to act on bodies “from nowhere”.

A famous example for an inertial force is the centrifugal force – an observer riding a merry-go-round needs to introduce that force to explain why he and all other riders are pulled away from the axis of rotation.