But you might be saying, Hey, come on Sal, what's going on? You just went through this, you said for most of most of human history, including my own personal history, this is what I observed. These guys say, Objects have a tendency to maintain their motion forever and the only way that you're going to stop them is if you act on it, or accelerate them, or change their velocity, so either their speed or direction some way, is to act on them with an unbalanced force. For over 2000 years, objects tend to stop on their own, if you want to keep the movement, apply a force. So the completly opposite way of thinking. Unless they're acted upon by an unbalanced force. And if their speed is zero, they'll maintain that restfulness. Maybe objects have a tendency to maintain their velocity, their speed and their direction. And their big insight, and it was very unintuitive at the time, Is that these three gentlemen said, maybe it works the other way. And they describe it in different ways, and Newton frankly gets the credit for it because he really encapsulates into a broader framework with his other Laws, and the Laws of Gravitation, which was really the basics of classical mechanics, and seem to describe, at least until the 20th Century, most of how reality actually worked. But these other two guys get at least as much credit for it because they actually described really what Newton's first Law describes, and they did it before Newton. And, indeed, one of these gentlemen is Sir Isaac Newton. But then these gentlemen show up in the 1600s, and you might be surprised to see three gentlemen here, because this is about Newton's first law of motion. And once again, this is completly consistent with everyday human experience, this is what we've all experienced our entire lives. And if you want to keep them moving, you have to apply some type of a net force to it. And this is why, for most of human history, probably pre-history, but we definitely know the ancient Greeks all the way until the early 1600s, so for at least 2000 years, the assumption was "objects have a natural tendency to stop." Objects. It seems like something will always stop. We've never in our everyday experience seen an object that just keeps moving on and on forever without anyone acting on it. And on the other side of things, if you want to keep an object moving, you have to keep applying a force to it. That if you do nothing to this moving object, on its own, this object is going to come to a stop. Human beings have always observed that if you have an object that is moving, so this is a moving object, traveling to the right here, that it seems to stop on its own. I've heard the Sun's gravity described like a rope or tether pulling on the Earth, with not so much force that we spiral in toward it (which would be bad), but enough force to keep us from spiraling away (which would be pretty bad, too.) There's just enough gravity to keep us going around and around and around. That's what happened to the planets going around the sun. When you get an object pulling you toward it, but you're already moving, you start moving in a curved path. But why in elliptical orbits? Well most of the mass in our solar system is found in the Sun and so the Sun has the greatest gravitational pull in our neighborhood in space. Andrew M explained pretty well to another question how the solar system got moving at its formation and its been going ever since. you need a lot of mass involved before we start to notice it. And the force is actually a pretty small one. I think it makes more sense to think of gravity as a pull, not a push, because gravity is a force of attraction that exists between any objects with mass. You know what he called the force that keeps planets (and anything else!) from flying off in a straight line once they start moving- gravity. It's not such a simple question, at all! In fact until Newton came along and figured it out, nobody had a really good explanation for your really important observation, that the Earth and other objects in space going around the Sun follow a precise and predictable path, or orbit.
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