Lesson 2: The Natural State of Motion and Newton's First Law
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. 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.
Newton's Impact 
Of all the branches of physics, mechanics has the most scientific and practical consequences.
- Mechanical and Structural Engineering
- Aeronautics and Astronautics
- Astronomy
Newton is generally considered the greatest scientist who has yet lived because he invented the theoretical scientific approach, invented a new mathematics (calculus) and because his work has had such an impact on future (i.e. present-day) science and technology. A further reason for his greatness is that his laws overthrew centuries of careful thought about motion by some very intelligent people (chief among them Aristotle).
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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A frame of reference with respect to which an object with no real forces acting on it will move with constant velocity, i.e. no acceleration. Newton's Second Law applies only in inertial reference frames.
This definition assures that F=ma will apply in an inertial reference frame. In fact, the first law may be regarded as a special case of F=ma in which F is zero, and hence a must be zero. It deliberately excludes accelerating reference frames such as a car going around a level turn - in this reference frame there are fictitious forces that appear to be acting on all masses, pushing them away from the center of the turn (centrifugal forces).An inertial coordinate system has no acceleration with respect to objects with no measurable force on them. Although its acceleration is zero, its velocity can have any arbitrary value. Hence any frame of reference moving with constant velocity relative to an inertial reference frame is also an inertial reference frame.
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