 A big force to the body that usually results in injury, so sudden acceleration, sudden deceleration usually means injury. For example, this is my daughter, that's my wife, here's my mother-in-law, anywhere you induce a sudden change in velocity you have big injuries. So if you look at car safety features, they're all about making the interaction time as long as possible. If you can spread out the interaction, you can do the deceleration with a smaller force. And that's what we have with crumpled zones. Here we have my mother-in-law's first parking lesson. Crumpled zones are deliberately engineered weak points in the car and they absorb the force. So rather than you coming to a sudden stop, you come to a halt over a larger period of time. Another thing that you would have is airbags. Here we have a car with no airbag, his face will be decelerated by the steering wheel. Do you see the way it bent over? Here we have one with an airbag and his skull is nestled in the welcoming bosom of the airbag. A shorter side here, you can usually tell if the car was being driven by a male or a female by examining the airbag afterwards. If it was a female, there tends to be a ring of lipstick in the middle where she goes oh! There was a time in the 80s where that rule didn't really hold because you had adamant gerand gerand and all the modern romance things. Everybody was wearing lipstick, even me for it, never mind. Every features in cars, everybody thinks, oh, seatbelts. The term for them is a real inertia seatbelt. It retards the inertia. We can take a look here, seatbelt on, seatbelt on, no seatbelts here. Look, oh, kiddies are launched forwards, there's a skull impact there. Now it looks like the babies, the kids are launched forward, but they're not. The car is brought to a halt and they just continue at their original velocity. So it's their inertia in action. The larger crash test dummies are held in place by the seatbelt, but just because you're wearing your seatbelt, that doesn't necessarily mean that you're safe or free from injury. What I want you to do now is look at what happens to the skull motion of this crash test dummy. Go back, do it one more time. Watch the skull of this one here. Do you see the way you get that overextension of the cervical part of the spine? Anybody like to guess how much your skull weighs in kilograms? Come on, it's not a rhetorical question. Ten kilos is too much, seven kilos is too much. Five, four to five kilos, depending on how brainy you are, all right? That's a lot of mass at the top of your spine. So when the car comes to a sudden halt, the body is stopped, but the skull continues on. And that's what you get creating what's the name of that injury? Whiplash. Now there's two kinds of whiplash. The first kind of whiplash you get when the car is brought to a sudden halt. There's a second type where the car is already stationary, let's say at traffic lights, and someone coming behind you hasn't noticed that you've stopped and they drive into you. Under those circumstances, your car is shunted forward, your body with it, but your head, which was stationary, its inertia, wants to stay where it was originally. And that gives you a second type of whiplash. Now I scoured the internet for an example, and I couldn't see one relating to a car crash, but I did find this. Let me see now. Oh, go by. Where are we? There's my mouth. Come on. Now, watch the lady. She is attached to a speedboat. The lady is also attached to a parachute. She was supposed to go rising majestically into the air for an experience that she would never forget. Well, she will never forget it. That's for sure. Now, I'm going to show that again, but what I want you to notice is on the moment of acceleration, she's looking at the camera. And when she moves off, her body moves and for a second, her skull remains stationary and it's hinged back. The back of her head almost hits between her shoulder blades. Now that's the end of her holiday. The fact that she was turning sideways means she's activated another pair of muscles that will also be extended beyond their elastic limit. So she's going to be in a cervical collar at least five to six months if she's lucky. But let's just see it again. Look at the camera. That was linear momentum, the inertia where the motion is in a straight line. There's a second type of inertia, rotational inertia and things that are spinning like to stay spinning. We can see an example here. You spin the raw egg and then you stop it and if you let go, the fluid still rotates inside and it starts rotating again. It's simple enough. You can all do that at home. There's not a whole lot of science involved. You might say to yourself, well, big deal. What's the point? There's a very similar effect in the body that means if you didn't have that kind of rotational inertia, you wouldn't be able to stand up and walk outside the lecture theater because it's the basic principle in your vestibular system, your balance in the ears. Here we have your semicircular canals and you have three loops of fluid. One, two, three. And if you look at them mathematically, they're all mutually perpendicular. It will detect rotation in the x, y or z direction, well, the xz plane, the yz plane and the zx plane. And I need a couple of volunteers now. Come on, don't be shy. Thank you very much. One volunteer will do and I'll only do the one demonstration. Now, the vestibular system helps you with your balance and the way that you detect rotation. Thank you very much. What's your name? Andrew. Andrew. Do you just stand there for a moment? The way that you detect rotation is when you rotate, the bones in your skull go one direction and the fluid, the endolymphatic fluid in your semicircular canals remain stationary. You have a little hinge-like door there and when you have rotation, these things get bent and it activates the nerves. So you're all familiar with dizziness, yes? Come on over here. Sit yourself down. Now, you're all familiar with normal dizziness, yes? Anybody can get normally dizzy. Normal dizziness is where I just make you close your eyes and I spin you around and then when you open your eyes and I stop the rotation, your skull is stationary but the fluid in your semicircular canals is still going round and causing your cupular to deflect.