How Newton's Laws of Motion Work

By: William Harris  | 

A Brief History of Newton's Laws

It turns out that the great Greek thinker wasn't always right about everything.
It turns out that the great Greek thinker wasn't always right about everything.
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The Greek philosopher Aristotle dominated scientific thinking for many years. His views on motion were widely accepted because they seemed to support what people observed in nature. For example, Aristotle thought that weight affected falling objects. A heavier object, he argued, would reach the ground faster than a lighter object dropped at the same time from the same height. He also rejected the notion of inertia, asserting instead that a force must be constantly applied to keep something moving. Both of these concepts were wrong, but it would take many years -- and several daring thinkers -- to overturn them.

The first big blow to Aristotle's ideas came in the 16th century when Nicolaus Copernicus published his sun-centered model of the universe. Aristotle theorized that the sun, the moon and the planets all revolved around Earth on a set of celestial spheres. Copernicus proposed that the planets of the solar system revolved around the sun, not Earth. Although not a topic of mechanics per se, the heliocentric cosmology described by Copernicus revealed the vulnerability of Aristotle's science.


Galileo Galilei was the next to challenge the Greek philosopher's ideas. Galileo conducted two now-classic experiments that set the tone and tenor for all scientific work that would follow. In the first experiment, he dropped a cannonball and a musket ball from the Leaning Tower of Pisa. Aristotelian theory predicted that the cannonball, much more massive, would fall faster and hit the ground first. But Galileo found that the two objects fell at the same rate and struck the ground roughly at the same time.

Some historians question whether Galileo ever carried out the Pisa experiment, but he followed it with a second phase of work that has been well-documented. These experiments involved bronze balls of various sizes rolling down an inclined wood plane. Galileo recorded how far a ball would roll in each one-second interval. He found that the size of the ball didn't matter -- the rate of its descent along the ramp remained constant. From this, he concluded that freely falling objects experience uniform acceleration regardless of mass, as long as extraneous forces, such as air resistance and friction, can be minimized.

But it was René Descartes, the great French philosopher, who would add new depth and dimension to inertial motion. In his "Principles of Philosophy," Descartes proposed three laws of nature. The first law states "that each thing, as far as is in its power, always remains in the same state; and that consequently, when it is once moved, it always continues to move." The second holds that "all movement is, of itself, along straight lines." This is Newton's first law, clearly stated in a book published in 1644 -- when Newton was still a newborn!

Clearly, Isaac Newton studied Descartes. He put that studying to good use as he single-handedly launched the modern era of scientific thinking. Newton's work in mathematics resulted in integral and differential calculus. His work in optics led to the first reflecting telescope. And yet his most famous contribution came in the form of three relatively simple laws that could be used, with great predictive power, to describe the motion of objects on Earth and in the heavens. The first of these laws came directly from Descartes, but the remaining two belong to Newton alone.

He described all three in "The Mathematical Principles of Natural Philosophy," or the Principia, which was published in 1687. Today, the Principia remains one of the most influential books in the history of human existence. Much of its importance lies within the elegantly simple second law, F = ma, which is the topic of the next section.