 Biology students in the class, what's saliva for? The digestive system, if you talk to a biologist and say, oh well, saliva is the first part of the digestive process, the enzymes in the saliva, the digest proteins and carbohydrates. Complete rubbish. It does do that, but the primary function of saliva is to act as a lubricant to get the food from your mouth to the stomach. And I can show you very easily why that's the case with a little challenge. Do we have two volunteers? One, thank you very much. Preferably, I'd like to get a lady up to do this, as the other volunteer. I've given this talk six times in the last three days. I've never had a problem getting a volunteer. And all of a sudden, in Carlo, I need a second one. Come on. Alternatively, we could ask the teachers to nominate the most poorly behaved students. And while in the honour of Carlo is at stake, we need another volunteer. OK, maybe another bloke then, if there's no ladies. Well done, thank you very much. What we're going to do is the cream cracker challenge. And it's very straightforward. All you have to do is eat four cream crackers. But you have to do it in two minutes. And I have the cream crackers here, a fresh packet. Actually, when you open that and give yourself four, and four for the lady. I also have to do some ground rules. If you feel you can't go any further and you need a drink, there's a bottle of water for you each. But if you go for the water, your four fit the challenge. OK, so you lose. How many do you have? You have four and you have four. Right. It's OK. It was the 10 second rule. If it, if it. Yeah, all right. So the other thing is, people are often, sometimes what they do is they shove all four into their mouth at the same time. And that, that always ends poorly because people tend to gag. So if you think you're going to be sick, that's what that's for. So the bottle of water is for the end anyway. So you won't be sitting there with a mouthful of photos. So what we're going to do is we're going to start. It's a competition. See how many you can eat in two minutes. Five, four, three, two. The thing is, cream crackers absorb the saliva very effectively. So these people will be able to eat the first two quite easily. The third will be a struggle. But the fourth one will be almost impossible. It will be a bit like trying to swallow broken glass. Don't worry, go on far ahead. So we've got 30 seconds gone. How many do you have? Not even one eaten yet. OK. So the, it's the battle of the sexes here now. And I notice you're a very messy eater. You see, you're losing a lot of crumbs on the floor. That's a really sneaky trick. That one is. I was in Cork two days ago when we did this and the lady was losing. So she just went over and she grabbed his cream crackers and he wouldn't let me finish the last two. You leave his crackers alone. All right. So we've got 35 seconds left. How many cream crackers? There's still two left. 20 seconds left. Now that's one thing. It's, it's one good thing getting it into your mouth. It's a whole different business getting it swallowed. One, like very much. You can see you never really thought about saliva as a lubricant, but there it is. It's a vital part of the whole process. Right. Now that you've eaten, thinking about the thoracic cavity, you've got your lungs inflating. You've got your heart pumping. Everything's moving and it's rubbing up against each other. You have to make sure there's no friction that's going to cause any kind of abrasions. So you can see on the outside of the lung, we have this glassy membrane whose sole function is to act as a lubricating boundary to stop any problems. So the body is a very sophisticated engine where friction control is concerned. Back to mechanics. If you do mechanics, almost the first thing you learn about is Newton's laws of motion. And his first law is probably the most important one. Objects at a fixed velocity prefer to stay at that velocity and will resist efforts to change. This resistance is called inertia. And we can see an example of it here. With a skid, it's a funny thing because you're relying on frictional forces to keep you on the road. A particular type of friction called static friction. Even though the wheels are going round, it's static friction. But then if you start to skid, it changes the frictional force to a thing called kinetic friction. And the kinetic friction is always less than the static friction. And it's the ultimate practical joke that physics plays on drivers. The moment that the skid starts, that's the time when you need as much traction as possible, but the science takes away half of the force you need to regain control. So most skids, as soon as you started to skid, you're at the mercy of inertia. You're just going to continue in a straight line at a fixed velocity until something decelerates you. It might be a tree, a bridge, these kind of things. Newton's second law. Anybody know what Newton's second law is? No. Very simple. Force is massed by acceleration. Fear not. There's going to be no mathematics here. We can muck about with that and we get an equation like this. Now, I want you to think of another situation. Granny McGuigan is going to the shops in her car and she hits a tree. So she's decelerated from 60 kilometers per hour to zero. And the extent of her injuries is determined solely by this term here. This is the force that she's subjected to. That's her mass. That's her final velocity, zero. Her original velocity, one. The size of the force she's subjected to is determined by the interaction time. Anywhere where you have a very sudden change in velocity, you need a big force to do that. And if you apply a big force to the body, that usually results in injury. So sudden acceleration, sudden deceleration, usually means injury.