Galileo and Aristotle agreed on the same thing: what goes up must come down.
What they didn’t agree on is the why. Why do objects fall back down?
Their two perspectives, the Galilean and Aristotelian respectively, fiercely clashed in the late 1500s and early 1600s. It is one of the most popular and documented ‘feuds’ in the history of physics and helped inspire a paradigm shift of the 15th Century.
This article will revisit the two perspectives, mainly focusing on their differing views on free fall motion.
What is free fall motion?
Free fall motion is when a body moves under the influence of the force of gravity alone. An apple falling from a tree is a good approximation of this motion.
From the beginning, philosophers asked themselves these questions:
- What causes objects to move down?
- What is the nature of motion?
These questions inspired many interesting theories and hypotheses, a popular among being the Aristotelian view of motion.
Motion according to Aristotle
There are two types of motion according to Aristotle.
- Natural motion
- Forced/ Violent motion
An apple naturally falls from a tree. A block naturally slides down an inclined plane. A river always flows downwards.
All these are natural. An external agent isn’t necessary to make them happen.
On the other hand, it takes an effort to lift and throw a stone. It takes some work to push a block up an inclined plane. And fish must put up an effort to swim upstream.
All these motions are forced. They take some effort.
Motion and the four classical elements
Aristotle viewed motion through the scope of what we now call the four classical elements of air, water, fire, and earth.
The Aristotelian view holds that if these elements are left to settle, they would form layers simply by their inherent nature. This he called, natural motion.
Solid earth sinks through water and air forming the base, water sinking through the air to settle on top of the earth (or conversely air moving up through the water, as do bubbles), with fire existing at the top of the arrangement because it tended to move up through the air.
Thus, objects fall back to Earth spontaneously in an attempt to seek their natural place in the universe.
The fall of an apple, for example, would be explained this way:
Because the watery and earthly materials that make up the apple are both in a lower rank than air in the classical hierarchy of order. Then they would naturally descend through the air, to attain their place in the universe, which is at the ground (Earth).
With this view, Aristotle could explain:
- Why things fall
- Why heavy objects fall faster than lighter objects – An object with more earth, like a stone, would fall faster than an object with less earth, like a feather)
- Why force was necessary to keep an object moving in some cases.
To move an object against the natural hierarchy of elements, an effort would be necessary. Thus, making it a forced or violent motion. Throwing a body up in the air, pushing a box up a plane, and pumping water upstream are good examples of forced motions.
Related: Isaac Newton versus Aristotle
Galilean view
Galileo viewed motion through the lens of carefully designed experiments coupled with his remarkable intuition.
The Galilean view on free fall motion:
- Motion is more than the composition or nature of bodies. According to Galileo, mathematics can describe motion along with a few physical variables such as time and distance. Thus, free fall motion doesn’t depend on weight but rather the time of fall.
- In the absence of resistance, an external force isn’t necessary to maintain motion.
With the points above (and a few others), Galileo could explain:
- Why do heavy objects and lighter objects both fall at the same rate? This was contrary to common knowledge at the time. To prove his theory, Galileo apparently dropped balls of differing weights at the Leaning Tower of Pisa and they all reached the ground at the same time. There is also a brilliant thought experiment by Galileo to counter Aristotle’s notion of free fall.
- Parabolic motion. It was simply the combination of horizontal and vertical motion.
- Why objects fall. Rather than seeking their natural place in the order, they were all being pulled by gravity. Those that seemingly went up against gravity (like smoke) were due to the influence of the medium with which they were moving.
Galileo and Aristotle are two of the most influential thinkers in the history of physics. Although Aristotle has a bad rep today, his ideas helped explain motion in a consistent theory and supported by evidence.
Aristotle’s and Galileo’s ideas on free fall motion had some similarities but also many differences. They both appreciated the role of gravity and change in height as important factors, but they disagreed on how gravity worked and how it affected the speed of falling objects.
