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h1. Lesson 2:  The Natural State of Motion and Newton's First Law

\\  {excerpt}Newtonian Mechanics is named after Isaac Newton, whose Principia Naturalis (1684) represented the first scientifically correct description of motion and its causes.  Newton's first major insight was that the natural state of an isolated body (one with no forces on it) is motion with constant [velocity]. This contradicted more intuitively appealing earlier theories of motion in which the natural state of motion was at rest.{excerpt}  It also represented the first example of theoretical physics - the idea that one should start from a few hypotheses consistent with experiment, expand on their consequences, and compare the resulting predictions with reality using the most accurate experiments possible.  His starting point, the three laws of motion (called Newton's Laws), were not abstract hypotheses: the first two were summaries of Galileo's experimental work on motion, and the third is necessary for internal consistency. 

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Although Newton gave no reasons for why he picked his particular three laws and ordered them as he did, scholars speculate endlessly about his reasons.  It seems clear that his First Law was intended to directly contrast with the prevailing animistic views of motion.  In addition, the first law is an implicit definition of an inertial reference frame:


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