Galileo and Motion

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Before Galileo’s time, Aristotle was the god of gravity.

Seriously. Before Galileo came along, the question of how gravity worked was answered with another question: “What would Aristotle say?” Obviously, this method was faulty, since Aristotle was actually wrong about most scientific things he wrote about.

But Galileo began a tradition that would persist into the modern day. He’s credited for having performed the first true science experiments when he observed falling objects.

You could, of course, call Tycho Brahe’s night sky observations and Kepler’s correct application of mathematics to the heavens true science, but neither of them really performed experimental science. That distinction lies with Galileo.

Wait a second…so I thought Isaac Newton was the guy who discovered gravity?

Well, you’re right. The thing is, though, Newton may have figured out gravity, but he is quoted as having “stood on the shoulders of giants to get there.” And one such giant was Galileo, who’s known for more than just observing sunspots on the sun and mountains on the moon.

And no, Galileo didn’t invent the telescope and he wasn’t condemned by the inquisition for believing the Earth orbited the sun. But I’ve written about that already…

In Galileo’s time, it was believed that the universe was made up of four elements: earth, water, air, and fire. Have you heard of that before? I wouldn’t be surprised. These days, the classical elements are more often used to describe people’s personalities.


Aristotle held that each one of the classical elements had a natural tendency to move toward its “proper place.” The proper place for earth was—surprise—the Earth. Water settled just above earth, and air and fire each wafted out toward the heavens.

If you think about it, it’s actually an apt model of how the world works. It just epically fails at explaining why.

According to Aristotle, no force is needed to move an element to its proper place. But if you want to move it in a different direction—say, push a cart, or even so much as walk—you need a force.

Aristotle differentiated between natural motions, the movements of elements to their proper places, and violent motions, which are basically forced movements.

Galileo studied both of these. But instead of just endlessly debating and reasoning out principles that were treated as scientific, as so many before him had done, he decided to ignore Aristotle’s authority and see for himself how objects moved.

He did this by rolling balls up and down ramps.


If I asked you how you think this ball will roll if dropped, what would you say?

Do you think it’ll fall down at an even, constant speed? Or is it going to get faster as it reaches the tabletop? Or maybe, will it actually slow down as it moves?

Aristotle would have said that it’s going to move at a constant speed. After all, the ball is made of earth, and it’s returning to its natural place. There is no force there to make it change speed and move faster as it goes, and no force to hold it back.

Galileo let this ball roll down and discovered something incredible. Aristotle was wrong.

Everyone knew at this point that Aristotle was wrong about his model of the universe, with Earth in the center. But no one had any idea that he could be wrong about basic physics.

In fact…Galileo observed that a ball will speed up as it rolls down. Its speed will change, but its acceleration—defined in physics as any change in the motion of the object—will be constant.

That is, the change that’s happening to the velocity from any one moment to the next will be exactly the same from any other moment to the next.

One thing that’s important to realize in physics is that acceleration doesn’t mean the same thing it does in your daily lives. We generally think of it as speeding up, and deceleration as slowing down. Not true in physics.

Acceleration could mean speeding up, slowing down, or even changing direction. Is that crazy or what?

Here’s another thing Aristotle was wrong about: he thought that heavier objects will fall faster than lighter objects.


It makes sense that the “old idea” on the left would be true, doesn’t it? I mean, don’t heavier objects weigh more? They’re harder to lift, so isn’t it logical that they’ll fall faster?

Aristotle believed this for a different reason—he assumed that heavier objects were made of more earth and water, and so would be more eager to return to their proper places than lighter objects containing less earth and water.

Turns out…that’s wrong.

I know what you’re thinking. This blew my mind when I learned about it, and I’ll explain why it’s true in a post coming up soon—when I talk about how Newton explained why planets orbit in ellipses and not circles.

Here’s the thing. Galileo found that it doesn’t matter how big an object is (or, in science terms, how much mass it has)—all objects fall toward the Earth with the same acceleration, which he measured as 9.8 m/s2.

So, get this. A wooden ball and an iron ball will fall at exactly the same rate.

Why? Because, as I’ll explain later on, the acceleration due to gravity at Earth’s surface is always the same. Always, always, always. Absolutely no exceptions.

What about violent motions, then?

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Aristotle knew that objects could move away from their “proper places.” If they couldn’t, how could a human being so much as jump? How could mountains be scaled, if humans—which are made of earth—should just fall to their natural place?

Aristotle said that these motions happen because of a cause. He had no idea that this “cause” was a force, but he had the right idea. But he also said that an object can’t stay in motion away from its natural place without a cause.

How do arrows stay launched after they leave the bow, then? Aristotle said that air currents carry the arrow through the air.

Galileo proved him wrong by getting balls to roll away from what should have been their natural place.


In this experiment, Galileo rolled balls down the same ramp every time. This way, they gathered the same amount of speed every time. And for each experiment, he set up a different ramp for them to roll up.

According to Aristotle, there was no force on these balls after they left Galileo’s hand. None of them should have rolled up at all.

But as we know today, momentum carried them up—and gravity slowed them down.

Each ball rolled back up just as high as it had started out, no matter how steep the second ramp—because when the ramp was steeper, the ball actually had less distance to climb, so it could go higher.

But what would happen if there was no second ramp?

In this experiment, Galileo discovered that—if friction didn’t exist—objects would continue on moving horizontally forever, even though that didn’t return them to their “proper place.”

This was actually another nail in the coffin of the geocentric (Earth-centric) universe. People had argued in the past that if the Earth moved, as proposed in the heliocentric universe, then the moon would be left behind.

Galileo proved that the moon would continue on moving, regardless.

But Galileo never figured out why objects move the way they move. That discovery belongs to the famous Isaac Newton…

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