answered on 14 Mar 2019:
last edited 14 Mar 2019 2:05 pm
Great question! When a plane moves forward on the runway, the air that rushes past its wings goes a different path above and below the wing. Because the wing is slightly curved, the top path is longer and it means you get a pressure difference between the top and bottom, which lifts the wings up and with it the aircraft!
That’s why they have to reach a certain speed on the runway before they can lift off.
Because this force pulling the plane upwards is stronger than gravity when the plane is going at a certain speed, it balances out gravity and stays in the air!