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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. 

Newtonian Mechanics is extremely difficult to understand because it contradicts the intuitive sense of motion that you have developed in order to flourish in their everyday world.  All higher animals, including humans, have evolved primitive ideas and specific neural pathways to deal with motion because it is so important in our environment.  For example, your eyes will accurately track the simulated motion of a ball rolling down and then up a bowl-shaped path.  If the ball slows unnaturally on the way back up, your eyes will point to where the ball should be until your brain realizes that the ball has been delayed and your eyes jump tack to where actually appears.  This type of adaptation allows you to predict and avoid leaping tigers, falling branches, hurled missiles, etc. 

You also have ideas about motion, called phenomenological primitives, that you regard as true without further intellectual justification and upon which you base your everyday thinking.  Examples are "a bigger force causes a larger motion", "heavy objects have more friction with the ground than light objects"  [??elaborate on Dicessa's work].

One phenomenological primitive concerns the natural state of motion of an object.  All theories of motion start here: what is the motion of an isolated body,i.e. one with no forces applied to it.  In the Aristotlean view, the natural state of an object is at the center of the earth, consistent with his view that gravity was the only "natural force". (The modern result that gravity does indeed vanish at the center of the earth lends it ironic support.). 

In the animistic view of motion in vogue in the Middle Ages, the natural state of an object was at rest with respect to its surroundings.  Ask yourself, don't you intuitively believe this?  Look around you.  Objects on your desk and around the room are indeed stationary with respect to their surroundings - even on a fast moving airplane (driven at constant speed by the force of its engines), objects inside the craft are at rest with respect to their surroundings.

Newton's First Law of Motion directly contradicts this:

If an object is moving with no force acting upon it, then it will move with constant velocity. Note that velocity is a vector, so this statement implies that the object will keep the same speed and the same direction of motion.  This directly contradicts the animistic view of motion in which the natural condition of a body is at rest with respect to its surroundings - the First Law says the natural state of a body is moving with zero acceleration, not zero velocity.

This law is so counterintuitive that it is easy for students to believe that physics professors live in an abstract world that is fundamentally different from the real world.  As poetically stated by one middle school student trying to get her mind around this dichotomy,
     Objects in motion remain in motion in the classroom and come to rest on the playground.
Simply put, adopting this two-worlds approach will not lead you to an understanding of Newtonian mechanics, which is the act!

IMPORTANTLY, You must modify your intuition to adopt Newton's first law as your phenomenological primitive.  When you see a sliding book come to rest, you must realize that this is unnatural and requires the action of a force such as friction.  When you push the book and it responds by traveling at constant speed (rather than accelerating), you must realize that constant speed implies no net force and search for another force acting opposite to your push (friction again).

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:

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.