 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 progressed 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.