 We've been describing motion using our normal everyday language throughout our whole lives. In physics we sometimes have to be a little bit more careful, so let's just take a few moments to recap some of the concepts we've introduced and look carefully at their definitions. So we normally describe motion in the real world in three dimensions, and that's a little bit tricky, so for this discussion we'll just talk about travelling along in a particular straight line. And as Vanessa's already discussed, what we need to do in order to describe that we need to be able to say where things are, so we need some kind of coordinates, so we have a zero in position, and then if we're at a particular position up here, we say that we are a distance x from that origin, and that position might change over time if we're moving around. So one of the things we have to be careful about to distinguish in physics is the difference between displacement and distance. Supposing I started at the origin and I travelled all the way over here, and then turned around and came all the way back to here, and so I ended up at that point there. Now if I'm there, the question you might ask is how far did I travel? And there are two perfectly reasonable answers to that. One answer says, well, I travelled that amount of distance, and then I travelled this bit back, and so I have to add that together, so I add those together, and that's how far I travelled. Another perfectly reasonable answer is, well, I only got that far. So that's how far I travelled. And those two answers are given different names in physics. The one where we have the long trip is called the distance you travelled, and this difference between where you started and where you ended is called the displacement.