AN INTRODUCTION TO NEWTONIAN MECHANICS by Edward Kluk Dickinson State University, Dickinson ND

        A historical overview
        A single push of an object resting on a rigid horizontal plane makes this object moving along a straight line. But after a while the object will stop. This simple observation is known for human beings for many thousands years. In 4th century BC Greek philosopher and scientist Aristotle postulated that keeping a body in motion with a constant speed demands a constant push. Or we may say, application of a constant force. But we have to admitt a vagueness of idea of force in this kind of statement. It does not tell us how to measure a force, then it does not have practically any scientific value. It took two thousands years and Isaac Newton to refine Aristotle's postulate and create fundamental theory of motion. Here we will make a first step in this direction.
        Actually it is not difficult to perform a following experiment. Take a hockey puck, set it flat on a well leveled big sheet of sand paper and give it a single slight push. The puck will not move very far. Replace the sand paper by concrete surface, a board covered with formica, and finally a smooth ice surface. It is clear that the same push will make the puck to travel longer distance as we are changing the surfaces. Probably everybody would say that such results are obvious because in each consecutive case we deal with less friction. A point here is to imagine yourself what would happen if there were no friction at all. Well, it looks like the puck should move with a constant speed across such frictionless surface. Then Aristotle was not quite right because if there is no friction, no force is needed to keep a body in motion.
        The last paragraph presents an example of abstraction method applied in science. Without having possibility to experiment with a frictionless motion we were able to deduce that keeping body in motion does not require any force, unless a friction, or possibly other forces trying to stop the body are present. Therefore bodies by their nature are trying to preserve (conserve) a status of their motion. Understanding how the simple experiments with hockey puck lead to such conclusion is very important as an introduction to Newton's laws of motion. Predesessors of Newton were not able to realize that if a force is needed to keep a body in motion this force is used only to overcome friction forces trying to stop the body.

the objectives of this lesson are fully achieved. If you have doubts try to read it once more concentrating on them, but do not try to memorize this text. physics is not about memorizing, it is about understanding.