 And the situation where the velocity is changing is where there's an acceleration. And just as the velocity is the rate of change of displacement, the acceleration is the rate of change of velocity. Now, acceleration doesn't have a scalar version because by the time you're talking acceleration, you're normally talking to a physicist, in which case you're normally talking about the vector quantity first time. However, we do use acceleration in day-to-day life. For example, you might say, I'm going to jump in my car and I'm going to accelerate. So I press the accelerator and that makes me go faster. And people are happy that that means acceleration. But what happens when I press the brake? That slows me down. Well, to a physicist, that's still acceleration because you're still changing your velocity, you're just changing it with a negative sign. And so you're accelerating backwards. So acceleration is a vector quantity. Another example of acceleration that typically is not understood as acceleration is where you're changing direction. Supposing you're in a car and you go around a corner, even if your speed says exactly the same, people might say that therefore you don't accelerate because you're not going faster or slower. But because your velocity is changing, because remember velocity has direction in it as well, that means you must be accelerating. So something has to happen in order for you to change that direction. And you'll find that very clearly if you try and turn a corner in a car and the road is very icy. If you don't have friction with the road, you don't get to change your velocity. And once again, of course, you can talk about an instantaneous acceleration or an average acceleration, exactly the same sense as you do for velocity. An instantaneous acceleration is the acceleration defined over a short period of time. And an average acceleration is the acceleration defined over some large period of time.