 So let's go met these laws of nature first. Let's do laws of nature, laws of nature. Now you needn't have gone to university or to college to know Newton's three laws. Laws of motion, those are laws of nature. And we can have a look at Newton's second law. Let's say Newton's second law. You can say, well, if you call in A and we can make this a vector, in A a vector. I mean you just learn that, that is just the way it is. But how can you come up with this as a physicist? Well, you don't know that this is the relationship. You don't know really what a force is, but you see acceleration. You notice when you get a layer that if you drop something, it falls, it goes fast and fast. If you roll a ball down an incline, it goes fast and fast. There's some acceleration. And so you know that acceleration exists. It's a vector because it has quantity, it has size and it has a direction. And you think to yourself, let's make a proportionality. Let's make it proportional to something and we call that something, a force, which is also a direction. You've got a choice. You can say, well, let's make it inversely proportional. Let's make it proportional to the square, the thing that we created. We call it proportional to the square of the acceleration. There's lots of things you can come up with. But let's start simple and we just say, well, it's just proportional. And if that's a linear proportionality, remember it's linear, that's to the power 1, that's to the power 1, it's not inverse. This is linear proportionality. You can put in, let's just write things the proper way round. So you say, if it's proportional to the acceleration, now you can put in a constant of proportionality and let's call that inertia, inertial mass. You say, that looks good, let's try and test this. I have an effect and I have a cause. And I can change that cause. I can push something and I can now change this proportionality and see if this constant of proportionality see if my effect is different. I can now go out and test this. And if I do lots of tests and I can find no experiment that proves otherwise, I accept this as a law of nature. That is a law of nature. And no one has ever found within the realms of classical mechanics. Now if you're interested in physics, if you know anything about it, you'll know that this is just a very special case. We're talking Newton's laws of the special case in various circumstances, say for instance of general relativity. And you know that's not completely true, that there's a warping of space time due to a large gravitational mass, or a large mass. But within the realms of classical mechanics, no experiment has ever found anything different and we now call it a law of nature. Now we go to the first law. The source actually should be called Galileo's law. He stated this and then some suggest, well, you just set this equal to zero. Remember acceleration is dv dt. And that's zero, but anyway that the derivative of something is zero, zero vector, well, zero vector, anything that at the first derivative of zero is if that velocity is a constant. The constant's either zero, so the object's standing still, or it's some value, so there's linear motion in the same direction at the same velocity. We call that Newton's first law. Let's not need this first law. That's a derivation of the second law. If it's just simply a derivation, why do we call it a law? It is a law for specific reason because it sets the existence of an inertial reference frame. And an inertial reference frame is something in which equilibrium holds. So you've created this law, you've created this circumstance. You call it the first law. Under those circumstances, a type of space in which equilibrium holds. In other words, you need no cause to give you the effect of something standing still or going at constant velocity. You need no interference with that. If you create a system, a universe like that, that is a law of nature and it has nothing to do with this derivation, it stands completely on its own. In the next section, we'll have a quick look at Newton's third law.