Laws of Motion

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Sir Isaac Newton physics is based on the fundamental concepts of mass and force. In his book “Principia,” written in 1685, Newton investigates the properties of different types of forces. According to The Cambridge Companion to Newton (Pg 57), the most important of these forces are those which produce accelerations or changes when a body is in motion or at rest. In his book “Principia” Newton separates these forces into three main categories: impact or percussion, pressure, and centripetal force. Newton also mentions a force of inertia which according to The Cambridge Companion to Newton (Pg 57) it is not a force at all according to the perspective of dynamics. In “Principia” Newton refers to mass as “quantity of matter,” this is determined by density and volume. He also refers to force as an “action” that alters the “state” of a body. To keep this short, most of Newton’s work in Physics contains his three Laws of Motion:

“Every body preserves in its state of being at rest or of moving uniformly straight forward except insofar as it is compelled to change its state by forces impressed.” (Cohen & Smith Pg 64)
In American English this means that a body remains in constant motion or at rest unless an external force is applied. That is the reason why the first law is usually referred to as “The Law of Inertia”.

“Change in motion is proportional to the motive force impressed and is directed along a straight line in which this force is impressed” (Cohen & Smith Pg 65). To keep this simple, F=ma, which in terms refers to the force being proportional to the mass and acceleration of a system.

“To any action there is always an opposite and equal reaction” (Cohen & Smith Pg 68). This law, as Newton mentioned, applies specifically to collisions and means that for every action force there is an equal reaction force in the opposite direction.