Newton's three Laws of Motion revolutionized the sciences and are important today as some of the most basic principles of physics.
Isaac Newton's three laws of motion published originally in Philosophiae Naturalis Principia Mathematica in 1687 were some of the first quantitative assessments of physical motion, and are among the bases of classical mechanics. They describe three elements central to elementary physics: inertia, force as it relates to momentum, and reciprocal force between bodies.
Sometimes called Galileo's Principle because of its relevance to celestial movement, Newton's "Law of Inertia" indicates that a body of matter remains either in a state of rest or in motion in a straight line at a constant velocity until it is affected by an outside force. This was revolutionary in Newton's time because pre-Newtonian physics were mostly based on the judgments of Aristotle, who believed that the natural state of an object was a state of rest. In everyday life, this would be observed as true; for instance, a ball rolling across a flat surface will eventually come to a stop. This is, however, the result of friction, the contact of the ball against the ground. In a theoretical frictionless environment (such as a vacuum without gravity), the ball would continue to move indefinitely. This observation helped to consolidate our understanding of the movements of celestial bodies with practical physics.
Newton's second law, his "Fundamental Law of Dynamics", indicates that the acceleration of a body, given that its mass remains constant, is proportional to the amount of force placed upon that object, and that the change in momentum will occur in the direction of the force applied. This rule is important especially because it was perhaps the first formal statement that bodies interact by trading momentum between one another, which leads us to Newton's Third Law.
Newton's "Law of Reciprocal Actions" indicates that the force exerted by body A upon body B is also necessarily exerted by body B upon body A. For instance, a ball moving to hit another ball might knock the other ball into motion, but in doing so it may become motionless; a car that hits a brick wall at 40mph will be faced with the same force as though that brick wall were actually flying at the car at 40mph, and so on. While this may seem intuitive, before Newton's time it was not understood to be a principle of physics. The other implication made here is that momentum is conserved, meaning that it does not really disappear (except visibly, as a result of friction, which is actually the same process occurring on a smaller level). Energy does not disappear in a physical interaction, it is only exchanged.
Newtonian physics represented something a reinvention of that branch of science, which in Europe had lingered in a focus on the Classics (in particular Aristotle) with little interest in amending the principles that had already been put forth. Newton was hailed by scientists and laymen alike as "new blood" in the field, and his three laws and other works lent a great deal to the movement to make science truly quantitative, rather than founded on idle observations.
