Aristotle was perhaps the first in the Western tradition to look at mechanics in any sort of structured way. A philosopher, rather than physicist, Aristotle thought about the way objects interact with each other, particularly their motions.
One of the ideas to come from Aristotle’s work is that objects “like” to remain at rest. This seems rather reasonable – put a book on a table and it remains still, push it gently and it will move until you stop. This begs the question, though – what happens when we throw ad object? Our hand stops pushing, but the object continues to move. Likewise when we roll a ball – we release the ball and it continues to move. Aristotle’s answer was impetus.
When an object is moved by another (your hand, for example, throwing a ball), it accrues impetus. When the mover stops acting upon the movee, the impetus it accrued whilst being acted upon is used to continue the motion. Under this model, we would expect objects to exhibit straight-line trajectories (see figure one) rather than the parabolic trajectories (see figure two) we see when we throw an object.
Figure 1: A straight-line trajectory of the sort predicted by Aristotle’s theory of impetus. |
Figure 2: A parabolic trajectory of the sort predicted by classical mechanics. |
A second idea of Aristotle’s is that heavier objects fall faster than lighter objects. It does, at first glance, seem rather reasonable but it is, like the idea of impetus, quite easily shown incorrect.
The Aristotleans didn’t bother to take observations or do experiments to support their beliefs and most of those that came after them were content to trust Aristotle. Thus for more than 100 years, our understanding of mechanics was fundamentally flawed. It is the resolution of this flaw that brings the next major milestone in mechanics: experimentation.
During the 16th and 17th centuries, Galileo Galilei and other became amongst the first physicists when they used experimentation to confirm and reject their ideas about the motion of objects. Among Galileo’s more famous experiments (though the story is now considered to be untrue) is his dropping balls of varing mass of the Leaning Tower of Pisa by which he showed that, contrary to Aristotle’s account, the speed of a falling object is independant of its mass. It is precisely this power – to overturn wrong ideas, even if though they have been believed true for centuries, and to suggest a more complete understanding – that makes experimentation so central to all of the sciences.
This experimental focus was not the last development in the physics that we’ll be looking at, though it did help pave the way for it. This next and final (for our purposes) leap was due to Newton – using mathematics to describe physics. After that, classical mechanics was essentially complete, with “only” quite a few decades of improvements and polishing before the introduction of relativity and quantum mechanics. It is physics at this level, the state of the art of classical mechanics circa the mid 19th century, that we’ll be studying in this course.