 Newton's first law. Before Newton it was thought that objects had a natural state of being at rest. That means you had to push on it to get it moving. You had to keep pushing on them to keep them moving. And once you stopped pushing on it, it's going to come to rest. This view could be summed up as forces cause motion. Newton had a little bit of a different view. He said that forces cause changes in motion. So if you're not moving, you need a force to get it moving. If you are moving, you need a force to stop it moving. And without a force, it just does what it was doing before. Now this comes down to a concept called inertia. It's a property of matter that it resist changes to its motion. And this concept existed before Newton. He kind of formalized it. So when he formalized his first law, it starts out by saying a body will tend to remain in its state of either at rest or moving in a straight line at constant speed unless acted upon by an outside force. Now when we're reading Newton's first law, you're going to see it phrased a little bit differently in different textbooks. And it's important to remember that this is a translation from the original Latin. So several of these different ways to phrase it all come out to be the same thing. You might hear a body at rest stays at rest and a body in motion stays in motion unless acted upon by an outside force. Now when Newton first proposed his, a lot of people didn't think that sounded right. And that's because of the common experience of say pushing an object or shoving an object along the ground. You give it a push and after you stop pushing, it slows down. Well what Newton said was that there's really an unseen force called friction acting here. And if we could reduce the friction and get rid of that, then when we gave the object a push and it started moving, it would just keep moving. It wouldn't come to a stop. Now it's really hard to do that, especially in the ancient world. It's difficult even now. Newton's laws ended up prevailing because he didn't just put forward a philosophy. He was able to quantify those effects, which led to predictions and experiments to verify that what he was saying makes sense and works in our physical world. So that's an overview of Newton's first laws. We'll look at the quantification of that when we move on to the second law.