MIT 8.01 Lesson 2: The Natural State of Motion and Newton's First Law
Lesson Summary
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. Newton's approach 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 using mathematics, 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 a description of the nature of force that is necessary for internal consistency.
Learning Objectives
By the end of this Lesson, you should be able to:
- State and explain your own words Newton's First Law of Motion.
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).
Animistic vs. Newtonian Worldview
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 experience more friction with the ground than light objects", etc.
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 rest at the center of the earth, consistent with his view that gravity was the only "natural force".
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.
The Newtonian view, set down in Newton's First Law of Motion directly contradicts this:
Newton's First Law: 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.
Or perhaps the contradiction is not so direct. Newton's Law really describes the motion of an object in the absence of surroundings. The important advance here is that Newton realizes that the surroundings must act to bring the object into a relative state of rest. Newton asks us to consider carefully how much influence a moving object's surroundings exert on it, which leads to a quantitative study of interactions and thus to Newton's Second and Third Laws.
Newton's First Law is so counterintuitive that it is easy for students to believe that physics professors spend too much time 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,
Importantly, over 300 years of scientific discovery confirms that 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).
Inertial Reference Frames and the First Law
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:
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.