 Now let's look at the concept of isolated systems from a momentum perspective. Now whenever you start talking about a system, the first thing you have to do is carefully define which objects are in the system. And we do this because we need to define the internal interactions, which are those interactions between objects in the system, and external interactions, which are between objects that are inside and a second object outside. And when we have non-isolated systems during the time of interaction, there's an external force that exists. And those external forces don't balance each other out. So let's give a few examples so you can picture it a little better. Let's say I have an outside applied force. So I've got a block and somebody reaches in and pushes on that block or tugs on that block, but that person is not part of the system. Or you could have an object hitting a wall. Even if you consider the system to be the object like the ball and the wall, the wall can't be isolated from the ground because there's always a force on that wall holding that wall in place. Or an object sliding across a rough surface. As it's sliding across that rough surface, you're going to have some friction on it, which is going to be an outside force. Or if you have an object in free fall or projectile motion, in that case you have the outside force of gravity acting on the system. Of course, there's many others as well, but the concept here is there has to be an external force which is not balanced out acting on the system. So now let's think about isolated systems. During the time of interaction, there's no external forces. Or at least they all balance out. For example, you could have an object sitting on a smooth surface, in which case it has a weight and a normal force, but those exactly balance each other out. Or you could have plenty of internal forces. So this statement that there's no external forces doesn't discount that there could be internal forces acting on them, but they balance each other out. Now, how do I know if the internal forces balance each other out? Well, we can come back to Newton's third law. Newton's third law deals with the fact that forces are interactions between objects. And that means these interactions have an action-reaction pair. So if I have two objects that are pressing up against each other, they each exert a normal force pushing the two objects apart so that the objects don't go through each other. And Newton's third law says that these two forces are exactly equal and opposite to each other. If both of the objects are in our system, that means the forces are internal and they balance each other out. So what does that mean for the net force? If all the forces are internal, then there's no net force on the system, because the internal forces always balance each other out by Newton's third law. So that means the impulse is zero. If I have no net force, I can't have an impulse. And that means I've got no change in the momentum. Now this is of the system. Each individual object in the system might have its momentum change, but the momentum of the system doesn't change. So this leads us to the concept of conservation of momentum. For the whole system, momentum doesn't change. As an equation, that means the change in momentum is zero. But this can also be written as the initial momentum is equal to the final momentum. This is the total momentum of the system. So again, this initial momentum is the initial momentum of all of the objects in the system added together. And for the final momentum, that's the final momentum of all of the objects in the system added together. And that total momentum does not change before and after the interaction. So that defines what we have for isolated systems.