Newton's First Law

Newton's First Law of motion states that an object will remain at rest or in uniform motion in a straight line unless it is acted upon by an external force. It may be seen as a statement about inertia, that objects will remain in their state of motion unless an external force acts to change the state of body. Any change in motion involves an acceleration, and then Newton's Second Law applies; in fact, the First Law is just a special case of the Second Law for which the net external force is zero.

Newton's First Law contains implications about the fundamental symmetry of the universe in that a state of motion in a straight line must be just as "natural" as being at rest. If an object is at rest in one frame of reference, it will appear to be moving in a straight line to an observer in a reference frame which is moving by the object. There is no way to say which reference frame is "special", so all constant velocity reference frames must be equivalent.