Classical Thinking in Physics and Newton's Laws of Motion

The discovery of the Newton's Laws of Motion provide many insights into how scientific discovery occurs - there are experiments and observations, consideration of unknown or untested variables in the experimental observations, theories knitted together from results and analogies in other systems, more experiments and use of different experimental paradigms, and finally discussions (and often disagreements) with others until one arrives at a consensus.

It was Aristotle who first noticed that masses of heavier weight tended to move more quickly in proportion to their size. This didn't make sense to Galileo Galilei, though, because he knew that projectiles like cannons didn't move in straight lines, but rather in straight curves. There has been a popular story (perhaps perpetuated by Galileo's biographer) that Galileo dropped two canon balls (one 10 times heavier than the other) from the Leaning Tower of Pisa to refute this law of Aristotle. There still remains some controversy over whether Galileo was actually able to show this, but he did try experiments using inclined planes (rolling balls down planes at different heights) to better control for the rates of acceleration and quantitate his results (in a vacuum or no air resistance), items of different mass should fall at the same rate. A page from one of his notebooks can be seen below.



The year Galileo died was 1642, the same year that Isaac Newton was born. Newton's great insight was to realize that a single law might be able to explain an apple falling from a tree, the curving movement of a cannon, and motions of the planets.

Putting his observations with the concept of Universal Gravitation, Newton imagined that if a cannonball were shot horizontally and fast enough from an iman imagined mountaintop, then it might actually orbit the earth. For an animation of Newton's cannon on a mountain, click here



Newton - What Really Happened with the Apple
PBS: Galileo's Experiments
The Galileo Controversy
Galileo's Notes on Motion