 Now in the previous video, Joe explained how natural philosophers came to understand the law of inertia, that an object will continue moving at a constant speed and direction unless it's acted on by forces. Now there's a lot to pick apart from that statement. What is a force, first of all? How can we quantitatively describe the motion of an object? And how is that motion going to change when the object is acted on by a force? So we're going to start by showing how the motion of a large complex object can be broken down into simpler parts. I want you to think about throwing a water balloon up into the air. The balloon goes up and down and it probably spins around and it changes its shape a little. Now these are the three types of motion that can occur in any situation, from an explosion to a flower opening to a satellite travelling through the solar system. To completely describe the motion of the water balloon, we should specify exactly how each point of the balloon moves through space and time. For example, if we choose the knot of the balloon, it's going to follow some curly path that's going to be really difficult to write down an equation for. But it turns out that there is a special point associated with every object that moves along a much simpler path than any other part of the object. And this point is called the centre of mass. It is much, much simpler to describe how the centre of mass moves through space. And then if we really need to, we can describe how the rest of the object moves relative to the centre of mass. In most situations, just knowing how that centre of mass moves is enough. And what this means is that we just have to track how a single point moves in time.