 Imagine you are standing on a tennis court and someone has found one of those machines that fires tennis balls that are sometimes used to practice. And they are firing a steady stream of tennis balls at you. Each tennis ball hits you and then bounces back off. So there's not just one machine, but hundreds of machines all firing an endless stream of tennis balls at you. That would be pretty serious. That many tennis balls bouncing off you would probably knock you flat and knock you over. They would apply a big force to you. Now that might seem like a rather silly example, but actually as you sit here watching this video, hopefully with a smile on your face, you are being bombarded right now. Not by tennis balls, but by air molecules. Trillions upon trillions upon trillions of air molecules are bouncing off your skin right now in all directions. So what you might think? Air molecules? That's pretty pathetic. They are so small they don't count. True, but there are an awful, awful lot of the air molecules hitting you and as we have seen they are moving extremely fast. It turns out that they are applying a very, very substantial force to you right now as you sit here. That force is several tons of force over your entire body. It's like underground with a car parked on top of you. It's an absolutely incredible force. Air pressure is normally measured in pascals. That's the unit. And a pascale is a force of 1 Newton per square meter. And the pressure of the atmosphere around you is about 100,000 pascals. So if you're, I don't know, say 2 meters tall and say 50 centimeters wide then your area on one side of you is going to be about 1 square meter. So the force is going to be 100,000 pascals applied to you. That's about 10 tons of force applied to you. It's absolutely enormous. Just the impact of all these little air molecules bouncing off your front. Now why doesn't it just push you over? Well of course there's that pressure coming in on one side of you but there's also the same pressure coming in on the other side of you, the air on the other side too, and so that balances out. But wouldn't it crush you? Well if you consider your skin and zoom in close, it's made of cells and each cell has water inside. And the water inside is at the same pressure as the air outside. The water molecules in there are banging into you. So the pressure from the outside or the air molecules banging into is perfectly balanced by the pressure from the water molecules inside. And likewise you might have veins and blood flowing through it and the air in there is also moving at the same speed and bashing into you. You've got your lungs with air inside there all bashing into you. So it's kind of strange. In the middle of this absolutely staggering force, 100,000 newtons of force for every square meter of area, we normally don't feel a thing because it's so perfectly balanced. The force from inside and outside balances perfectly. If it wasn't balanced, it would be in trouble. For example, if we connected a vacuum pump to our lungs and sucked all the air out, the pressure on the outside would collapse our rib cage. And particularly the air inside our lungs is at the same pressure as the air outside. Let's think of some examples of the sort of pressure at use. One example would be a spaceship. If you're in a spaceship, you've got your astronauts inside, and they've hopefully got air to breathe, but there's no air outside. So what that means is there's going to be a fairly substantial pressure pushing outwards. So your windows, for example, have to be bolted in. If they weren't, the pressure would push them out. So let's say you had a window that was 10 centimeters, 0.1 meter by 0.1 meter. And let's say the pressure inside your space station was the same as at the Earth's surface, 100,000 Pascals. There'd be an outward force on your window, which is equal to the area, the force is equal to the area times the pressure. So that would be 0.1 times 0.1 is the area times the pressure, 100,000 Pascals, roughly speaking, which comes out as about 1,000 Newtons. So that's the how strong the bolts would have to be to hold your window in. Another example would be a submarine under the sea. When you're underwater, the pressure can be much, much higher than at the Earth's surface. So you're at the bottom of the marionnas trench, the pressure could be thousands or even millions of times higher. So that means if you want to be inside it and not crush flat, you have to have a very strong skin because the air pressure inside would be, say, the normal pressure at the surface, but the pressure outside would be much bigger. You're being hit by so many fast-moving water molecules that it will crush your submarine unless it's very, very strong. This is actually the problem for most real submarines. They dive too deep, the pressure outside gets so big it crushes them, and therefore they collapse. That's why a normal submarine can't go to the bottom of the marionnas trench. You need a special extra-thick ward submarine to go down that deep because the pressure is so great. Another example of pressure is drinking. Let's say you have a cup of some soft drink and you want to suck it up. So you apply your mouth to the top here and you suck. But you can't really suck things up. What's actually happening is you drop the pressure inside, but the pressure outside of the atmosphere is still very big, and therefore that pressure pushes the fluid up to your mouth. So you're not really sucking the water up. The atmospheric pressure is pushing it. If you tried this in a vacuum, I wouldn't recommend it, and you sucked, nothing would happen. The water would just sit there. There'd be a vacuum inside here and a vacuum out there, and the gas wouldn't move at all. Another example is suction caps. Often, let's say you want to pick up some big sheet of glass in a factory. What you would do is you'd have cups on top, like rubber cups, and you suck the air out of them, and then they could be attached to ropes and they can pull the glass up, and that is in fact how glass is normally handled. What's happening here, you're not really pushing stuff up. You've got your piece of glass and you've got the suction cap on top, and the pressure inside is very low. What's actually happening is the pressure underneath is pushing up. So in fact, when you put a suction cap and pull a piece of glass or a piece of metal or something up, you're not actually pulling it up. What's happening is the atmospheric pressure is pulling it up. All you are doing is removing some of the pressure at the top, so it's no longer perfectly balanced. So that's pressure. Pressure is the force of billions of atoms hitting you at all points. The force applied to some area is equal to the pressure times the area, and the pressure applies in all directions.