 Imagine you're 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. See 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 will 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're so small they don't count. True but there are an awful awful lot of the air molecules hitting you and as we've 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 my 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 one 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 two meters tall and say 50 centimeters wide then your area on one side of you is going to be about one 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.