Centuries-old ideas about force and motion have an intuitive appeal that is enduring but oh-so-incorrect, as these simple experiments show.
Surely the Greeks weren’t the first to ponder the nature of the universe. Just think about it. Aristotle and his friends were having discussions about physics in the 350 BC time frame. But beer and wine were first created thousands of years before that. Thousands. It seems plausible that there were some other humans sitting around, drinking their fermented barley, and talking about why stuff happens. Right?
I suspect that we attribute these first physics discussions to Aristotle because some of his ideas survived with his successors and was passed down all the way to modern times. Aristotle’s ideas had a major impact on the philosophers and early scientists in the Renaissance era. In fact, his ideas about physics survived for an extremely long period—even though they were essentially wrong. Even today many people naturally tend to think of forces and motion in the same way as Aristotle, because they seem to make sense. I guess we should go over these ideas.
Of course I should start off by noting that I’ve never met Aristotle in person. I’m not an expert on the teachings of Aristotle, I just have some generic comments. If you want to complain, I’m fine with that.
The first important thing to consider about the Greek philosophers is that they were philosophers and not scientists. What is the difference? The Greeks approached ideas with some common and assumed truth and then deduced the details from that. For instance, take a heavy object and a light object (like a rock and a feather) and drop them. Which will hit the ground first? I think we can all agree that the rock will fall faster than the feather. So, this is something we assume to be true and the rest of the details can be deduced from that.
With that general framework, Aristotle had the following ideas about force and motion.
The first idea is that there are two kinds of interactions in nature. There are natural interactions and violent interactions. Natural interactions deal with the four elements: earth, wind, water, and fire. An object made of one of these elements wants to return to its natural position. Rocks are made of earth, so they want to get to the Earth. If you let go of a rock, it “falls” as it moves to where it belongs. Fire wants to get to the place of fire—which I think is up, so fire goes up. Oh, there is one other important idea about the natural state of objects. An object in its natural place is also at rest and not moving. Objects “want” to be at rest. Most objects desire nothing more than to be left alone.
The other interaction is a violent interaction. This is when you make an object do something it doesn’t want to do. If you push fire down, that’s violent and also not very smart (you will burn yourself). If you lift a rock, you move it away from the Earth. Finally, if you push a rock to the side and move it—that’s violent. You are making the object move when its natural state is at rest.
This means that according to Aristotle’s physics, you need a constant force to move an object at a constant speed.
Now let’s skip forward in time from Aristotle to Galileo. Yes, that same Galileo guy who used a telescope to support the idea that the Earth orbits the Sun rather than the other way around. But he also did some experiments with forces and motion. Yes, actual experiments. Galileo didn’t start with some assumed truth and then deduce the details. Instead, he set up an experiment to obtain results and then build an idea from there.
OK, it should be clear that experiments are difficult. How do you look at the motion of a falling object? Does it fall at a constant speed or does it increase in speed? If you take a pencil at arms length and let go, the pencil does indeed fall. It falls so fast that it takes less than 1 second to hit the ground. In this super short time interval, it’s pretty much impossible to see if it’s increasing in speed or just falling really fast.
Oh, I can hear you now. “How hard could it be? Just use the high-speed camera on your smartphone—boom. Easy.” But Galileo didn’t just not have a smart phone, he didn’t even have a nice clock. He had to make do with the tools he had. Physics can be tough.
But there is a creative way to look at the motion of a falling object. The trick is to not let it fall and instead let it roll down a ramp. How does a ramp help? Let’s examine the two extreme cases of a ball on a ramp. What if you have a horizontal ramp? In that case, a ball would just sit there and not move (if you released it from rest). This is indeed slow enough for even the casual observer to measure. The other case is a completely vertical ramp. A ball rolling down a vertical ramp would be the same as a dropped ball. So we can just slightly tilt the ramp from the vertical orientation and it’s mostly like falling. Or we could barely incline the ramp so that it still moves sort of like a falling ball, but much slower.
That is exactly what Galileo did. He rolled a ball down a ramp. Here’s what that looks like.
Notice that I marked the ramp in distance increments of 20 centimeters. That will let us see if the ball does indeed roll at a constant speed or if it increases in speed (although it’s pretty easy in this case to just see it with the plain human eye). If the ball moves at a constant speed, it will take the same amount of time to travel the same distance. OK, let’s do this. Here are the times it takes for each interval.
It’s clearly not moving at a constant speed. The time for each interval keeps getting shorter, indicating an increase in speed. But what if we take this to the extreme case? What if the ramp is a little bit steeper? That’s not an impossible experiment to perform—but you would find that the times still get smaller in each successive distance interval. But what about a vertical ramp? Yup, it just seems logical that a falling object increases in speed.
This is a big deal. If a falling object doesn’t move at a constant speed, it says that the gravitational force must make the object CHANGE its speed. Note: change is the key word here. But doesn’t this bring up another big problem? If forces change the motion of an object, then why does an object slow down and stop after you push it?
Let’s go back to the ramp with the ball. If you have a completely horizontal ramp—then yes, the ball slows down as it rolls. But what if the ramp is just mostly horizontal? What if there is a super tiny tilt to the ramp and you give a ball a push? Here, check it out.
Notice that after the ball is pushed, it seems to travel with a constant velocity. This seems to indicate that if we could make a perfectly smooth horizontal ramp, the ball would travel at a constant velocity. This is a big deal—especially since Aristotle said that the natural state of motion is to be at rest.
OK, that’s a lot of stuff. Let’s go over the important bits.
- Aristotle says (without evidence) that the natural state for an object is to be at rest. If you push on an object with a force, it moves at a constant speed. If you stop pushing, it stops.
- Galileo decides to do some experiments. He rolls a ball down a ramp and shows that it increases in speed.
- If you get a barely inclined ramp and give a ball a quick push, it will continue to move at a constant speed.
- Aristotle was wrong. If you leave an object alone, it moves at a constant speed.
It turns out that not only do Aristotle’s ideas about force and motion make sense, they are very common ideas held by very normal people. Perhaps the biggest problem is the presence of an unseen force—the force of friction. When you push on a block that sits on a table, it’s pretty easy to see that your hand exerts a force. When you take away your hand, you take away the force and the block comes to a rest. However, it’s not the lack of a pushing force that makes it stop so much as the friction force pushing against the motion. Because the friction force is always there, we tend to forget about it. That’s perhaps why it took so long for someone like Galileo (and others after him) to change our ideas about force and motion.
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