Frame of reference

To describe the motion of an object, you must first be able to describe its position at any particular time. More precisely, the position has to be specified relative to a convenient frame of reference.

A frame of reference is an arbitrary set of axes from which the position and motion of an object are described. For example, a plane flying could be described in terms of the position of the airplane with respect to a big city, whereas a cyclist’s position could be described in terms of where they are in relation to one tree.

Frames of reference where Newton's Laws of Motion are valid are called inertial frames. They are frames where the Principle of Inertia is true. An inertial frame of reference has a constant velocity.