Introduction

Every object in the universe interacts, in some way, with every other object around it -- a cool breeze interacts with a tree, the tree in turn interacts with the Earth beneath it, and every living species is, in that sense, constantly interacting with nature around it. What sets a human's interaction with nature apart from an animal's is one extra quality: humans don't merely interact with the world, they also try to understand and explain what they observe about it, scientifically.

Among the questions curiosity has driven people to ask throughout history, few are as old or as basic as 'how do things move?' and 'why do things move?' -- and the search for real answers to these two simple-sounding questions has quietly shaped the path from early civilisation all the way to today's technological world.

Historically, two contrasting ideas about the connection between force and motion shaped the birth of mechanics. Aristotle argued, based on common-sense observation, that "force causes motion" -- implying that a continuous force is needed just to keep any object moving. About two thousand years later, Galileo challenged this with a careful thought experiment: a ball released from rest at height $h$ on a smooth inclined plane rolls down and, on a second smooth inclined plane, climbs back up to very nearly the same height $h$. As the second plane's angle of inclination is reduced (both planes kept frictionless), the ball must travel a longer distance to reach the same height $h$; when the angle is finally reduced to zero (a horizontal surface), the ball never regains height $h$ and would, in principle, continue moving in a straight line forever, since there is no force acting on it in the horizontal direction.

This directly contradicts Aristotle: an object can remain in motion even in the complete absence of any applied force. In everyday (non-ideal) situations, a rolling ball eventually does stop -- but this is correctly attributed to an opposing frictional force, never to the "using up" of some innate motive force, as Aristotle believed.

Newton went on to analyse the ideas of Galileo, together with those of Kepler and Copernicus on planetary motion, and distilled them into three precise, quantitative laws -- the subject of the rest of this unit.