If you fall an object, it will fall. It’s a move we’ve all seen hundreds of times. We’ve also all seen a lot of the moon, which makes a full orbit around our planet every 27.3 days (as seen from Earth). Falling and spinning may seem like completely different types of moves, but they’re not! The same physics explains them both.
There is a famous story about Isaac Newton making the connection thanks to a falling apple. (It’s probably not true—but it power are.) Still, his realization is quite amazing, so I’m going to walk you through the whole process. It contains some concepts that people living today might take for granted, but building knowledge in this way is not trivial, and Newton didn’t figure it all out on his own. He built on ideas from Galileo, who studied the motion of falling objects, Robert Hooke, who investigated the effects of things moving in circles, and Johannes Kepler, who came up with ideas about the motions of the planets and the moon.
Let’s start with what happens to an object when it falls. In the third century BC, Aristotle claimed that a massive object will fall faster than an object of low mass. Sounds reasonable, right? That seems to fit what we see: imagine dropping a rock and a feather at the same time. But Aristotle wasn’t great at testing his theories with experiments. It just looked like it be meaningful that a heavier object falls faster. Like most of his philosophers, he preferred to draw conclusions based on armchair logic.
Aristotle also reasoned that objects fall with a constant velocity, meaning they don’t slow down or speed up. He probably came to this conclusion because falling objects fall quickly and it is very difficult to see changes in speed with the naked eye.
But much later, Galileo Galilei (who went by his first name because he thought that was cool) figured out a way to slow things down. His solution was to roll a ball down a ramp instead of dropping it. Rolling the ball at a very slight angle makes it much easier to tell what’s going on. It may look something like this:
Now we can see that as the ball rolls down the track, it increases in speed. Galileo suggested that during the first second of movement, the ball will increase in speed by a certain amount. It will also increase by the same amount of speed in the next second of movement. That means that during the time interval between 1 and 2 seconds, the ball will travel a greater distance than in the first second.
He then suggested that the same thing happens if you increase the steepness of the corner, as this would result in a greater increase in speed. That must mean that an object on a completely vertical slope (which would be the same as a falling object) would also increase in speed. Boom – Aristotle was wrong! Falling objects do not fall at a constant speed, but change speed instead. The rate at which the speed changes is called acceleration. On the surface of the earth, a falling object will accelerate downward at 9.8 meters per second per second.
We can write the acceleration mathematically as a change in velocity divided by the change in time (where the Greek symbol Δ denotes a change).