Newton's First Law of Motion

Newton's 1^st law is also known as the Principle of Interia. Its first known formulation dates back to Galileo, with a modification by Renee Descartes before arriving with Isaac Newton. Its straightforward meaning is that an object, free to move unencumbered by any kind of outside interference (like gravity or friction), will continue on forever in its current state of motion - even if that is being motionless. In this context, state of motion means moving at a given speed in a specific straight line direction. (Descartes' contribution to the principle of inertia is the idea of straight line motion, by the way.) This concept of continuous, unchanged motion is referred to as an object's inertia. The degree, or extent, of an object's inertia is measured through the property called mass.

Replay

A number of scientists have viewed the first law as being included in Newton's second law, discussed below, as a special case - that of motion with zero outside force. However, it can be viewed as important in its own right because it defines the observable property, unchanging motion due to inertia, that lets us relate the individual perspectives where Newton's second law holds true.

You see, everyone has their own unique and personal view of how things happen in the universe based on their own perspective and point of view - called a reference frame in the appropriate mathematics. But, to be able to communicate meaningfully with others, we all need to understand innately how to relate what we see (observations in our reference frame) to what others see (their observations in their reference frame). The principle of inertia permits us to establish a concept of relativity: the mechanical workings of the universe are the same in nature for me as they are for you. This Galilean postulate of relativity is valid for any pair of individuals in inertial reference frames, that is, reference frames where the principle of inertia is true. [Albert Einstein proposed an extension to Galileo's postulate of relativity in that all physical workings of the universe are the same in nature for everyone in inertial reference frames. He extended from mechanical workings of material objects to those that do not include ordinary material objects, like light. This became part of his Special Theory of Relativity.] Since Newton's 1^st law sets the stage, so to speak, for the other two, it can be understood as necessary on its own!