 Newton's third law. Newton's third law says that for every action, there is an equal and opposite reaction. Or we could state that as two objects interact, their forces of interaction are equal and opposite to each other. That brings our Newton's third law back into a description of the forces, which is what the Newton's first and second laws refer to. So we have forces, and those forces act on single objects, but they're always due to the interactions with other objects. So as A pushes on B, Newton's third law tells us that B pushes back on A. Mathematically, we can describe that as the two forces are equal in magnitude, but in terms of their vectors, they're opposite in direction. So that means we've got force pairs. One force of this pair is acting on each object. So when I had those two forces, the FAB was the force on object B caused by object A. Or you could think of it as the force that A causes on B. The FBA is the force on object A. So these two forces are on different objects, but caused by each other. So let's look at an example here. Let's say I've got two objects, an M1 and an M2. Well, our Newton's third law says that the two forces are equal and opposite, meaning that the force experienced by mass two, because it's being pushed to the right by mass one, is equal in magnitude but opposite in direction to the force experienced by mass one being pushed back by mass two. So in this particular interaction, the two masses are pushing each other apart. But just because the forces are equal and opposite doesn't mean the accelerations are equal. If I were to look at this example and look at the acceleration on this M2, well, the acceleration that it's going to experience, assuming that these are the only forces of action in our system, is the force divided by mass two. Similarly, if I look at my mass one, it has an acceleration, which is the force divided by mass one. But since mass one is potentially a much larger mass, in this case I've shown the sizes of the circles to also be an indication of how much mass they have. That means that I'm dividing the same number by a much larger number, and the larger mass is going to have a smaller acceleration. This one had more inertia. Now in extreme cases where you have one which is a much larger mass than the other, the acceleration can be so small that we hardly even notice it at all. Remember also that this is an isolated situation between two masses where there's the only one force of interaction between them. In other situations you might have other forces, each of which has a pair, but potentially with different objects. So that introduces the concept of Newton's third law. As we start applying Newton's laws to practical problems, we'll start to see how we use this law.