This is an airplane in flight, so what makes it stay up in the air? To understand the basic physics of flight, let's have a look at the properties of air, in particular, air pressure. Air is made up of many small molecules. Air molecules are made up of many tiny atoms. Air may appear invisible, but air has mass; therefore, it has weight. In the Earth's atmosphere, there are a multitude of air molecules. You can think of air as water, as it behaves like a fluid. Each time an air molecule comes in contact with an object, it puts pressure on it.

Let's have a look at this scenario again and discuss two types of air pressure. In the case of an object that is stationary, air exerts static air pressure on the object. For instance, how is it possible that an inflated balloon does not get squashed by the static air pressure? It's because the air inside the balloon balances the pressure outside. However, should the static air pressure outside decrease, the balloon will expand to fill in the space. Oops! If you force an object onto air, it's the object's energy that causes air pressure.

In this case, the object experiences dynamic air pressure. For instance, when you stick your head out of a moving bus, the wind resistance you feel on your face is dynamic air pressure. So, static air pressure plus dynamic air pressure make up total air pressure. What this formula suggests is that for a constant value of total air pressure, if dynamic air pressure goes up, static air pressure must go down, and vice versa.

If you observe air flowing through a tunnel like this, you will notice that static air pressure is applied to the walls of the tunnel, as represented here by the red arrows. For a straight and even tunnel, the static air pressure is equal throughout. But if you reduce the width of the tunnel and observe the airflow, something interesting happens. Inside the thinner section of the tunnel, the speed of the air increases. Consequently, the dynamic air pressure in this section increases. However, static air pressure decreases, represented here by the small red arrows.

So, in simple terms, the surface area where the faster airflow is has less static air pressure. Perhaps now you can see what we are getting at. The shape of the surface in the tunnel where the lower static air pressure looks similar to the top surface of an airplane wing. In part, this is how lift is created that allows the aircraft to fly, by reduced air pressure above the wing compared to the higher air pressure below the wing, as long as the plane is moving fast enough through the air.