Frame of reference: a coordinate system relative to which motion is described or observed

Inertial frame of reference: a frame of reference that moves at a zero or constant velocity; a frame in which the law of inertia holds

• for example, the earth is an inertial reference frame because with regard to our perception, it does not move

Non-inertial frame of reference: a frame of reference that accelerates with respect to an inertial frame; the law of inertia does not hold

Apparent weight: the magnitude of the normal force acting on an object in an accelerated (non-inertial frame of reference