 While we may know that gravity is the reason masses are attracted to one another, if you think back to a time before people really understood gravity, it's not immediately obvious that the force that's responsible for things dropping down to Earth is the same as the force that's responsible for keeping our moon in orbit around our planet, and our planet in orbit around our sun. It turns out that one equation can be used to explain these seemingly disparate phenomena, and it's the equation for gravity, or as it's more formally known, Newton's law of universal gravitation. So just when you thought you'd got through all of Newton's laws, here's another one. And this is the equation for gravity. In very simplistic terms, gravity is the natural attraction between different masses, so everything with mass feels some force pulling it towards every other mass. We will go into the exact form of this equation in the next video, but firstly I want to explore in a bit more detail how a mathematical equation is so important in understanding the physics of gravity. And to do this, I want to talk a little about physics more generally. Very broadly speaking, our understanding of physics can be regressed in a couple of ways. Firstly, and what you'll typically experience in your life, is physics through observations. And this physics can include experiments and labs, surveying the universe, collecting data, and fitting observations to models. But what physics through observations can't tell us is why the world is acting this way, and this leads us to physics through theory. Theoretical physics can include looking at symmetries and deeper patterns, understanding mathematical derivations and relationships, doing simulations, and gaining a more fundamental understanding of physics concepts. But to test our understanding of theoretical physics, we need to do observation physics, doing experiments and labs to test that the equations and relationships are giving us the answers that we expect. So if you go on to be a researcher or scientist, these observations and theories are how you would actually do physics. Some physicists are more experimental and do more observation physics. And some physicists are more theoretical and look at more fundamental concepts. And then there are physicists who sit somewhere in the middle between observation and theory. There are also those people who choose to go on and do engineering instead, and look at how they can apply physics in the real world. In this course, we're going to look at Newton's Law of Universal Gravitation, and this is an example of physics based on observation. There was no fundamental understanding that drove Newton to come up with his Law of Universal Gravitation. It was instead based on many observations and finding an equation that fit the many observations he had. This is in contrast to Einstein's more fundamental understanding of gravity, which is his general theory of relativity. This is a more advanced theory of gravity, and we won't be studying it in this course. So now that we have a better understanding of what physics is, how can we actually use all of this physics? As I mentioned earlier in this video, we will learn about Newton's Law of Universal Gravitation in the form of a mathematical equation. Now why is an mathematical equation so important to understanding physics, and why do we spend so much time learning about the equation for Newton's Law of Universal Gravitation? Well ultimately this equation is a tool that allows us to understand and predict how gravity affects the world. Lots of things are affected by gravity, and Newton's Law of Universal Gravitation allows us to model these effects. If we can quantify the rules that the world around us seems to follow, we can use these rules to manipulate our environment. For example we can work out how objects fall on the earth, how ramps affect the motion of objects, how a bank track lets a vehicle go faster, how objects in space pull and orbit each other, how far away a satellite should be to be in a useful orbit, and what happens when there are multiple objects pulling on each other. These are some of the topics we'll cover in the course, but there's a big difference between understanding the concept of gravity, which is that things with mass attract each other, and knowing how Newton's Law of Universal Gravitation works. Without a mathematical model, it's very difficult to make specific and testable predictions. But similarly, without understanding the concept of gravity, then Newton's Law of Universal Gravitation is just an equation with some random parameters in it. We need to use both physics concepts and equations to truly understand and use physics. In the next video, we'll be introduced to the form of this equation for the first time. But keep in mind that the equation is powerful because it can explain a lot of the physical behavior we observe, and ultimately, how our universe works.