In the US, it’s estimated that at any given moment, there are around 5,000 planes in the sky. They are so common, it’s easy to take for granted the incredible cultural access they afford us, and the unbelievable technology and engineering that goes into making them fly. Which raises the question: how do airplanes fly?

How does a metal tube weighing hundreds of thousands of pounds overcome the force of gravity and get 30,000 feet above the ground?

Airplanes, like everything else on earth, are kept on the ground by the downward force of gravity. In order to fly, airplanes must generate an upwards force stronger than gravity. This oppositional force is called lift.

While many parts of an airplane affect lift, most of the lift is generated from a simple shape, called an airfoil. Airplane wings are designed to create a difference in pressure between the top of the wings and the bottom of the wings. When a plane is parked on the ground, air molecules bounce off of the top and bottom of the wings in roughly the same amounts, with roughly equal pressure. However, when the plane is moving, the shape and angle of the wings splits the airflow in two directions: up and down. This forces more air molecules to bounce harder off of the bottom of the wings, which makes the pressure go up underneath the wings. At the same time, less air molecules are now bouncing off of the top of the wing, which lowers the pressure. In addition, the particles are moving across the top much faster than on the bottom, and as air speeds up, it’s pressure drops.

When the pressure underneath the wing is stronger than the pressure on top of the wing, the plane is able to take off into the air. As long as the upward force of pressure underneath the wings is greater than the downward force of gravity, the plane will be able to fly.

Airfoils are only able to generate this force when moving. If there is no motion, there won’t be any lift. It doesn’t matter if either the wing or air particles are stationary – a plane will still be able to fly on a windless day as long as it is able to generate motion.

Learn more about Lift and the forces of flight: NASA Glenn Research Center