How can a plane that weighs many tons stay in the air ?

The lift of an airplane comes from moving air. According to Bernoulli’s principle, air pressure decreases as it moves faster.

If you look closely at an airplane wing, you will see that it is curved on top and flat on the bottom. As the air flows past the moving wing, the air above the wing has to move farther because of the curvature. In order to do that, the air on top must move faster and therefore have a lower pressure. The pressure below the wing is higher, so it pushes the plane upward. Most people have investigated the Bernoulli principle without realizing it. Have you ever held your hand outside the open window of a moving car? If you hold it perfectly flat, you feel pressure along the front edge but no force up or down. Angle your hand slightly upward, though, and you feel a strong lift that tries to push your hand up and away.

Another common illustration of the Bernoulli principle requires a sheet of paper and a puff of air. Hold the paper in front of your mouth and blow above it. You might think the moving air would force the paper downward, but you observe just the opposite. The moving air above the paper exerts less force than the stationary air below it and the paper moves upward.

PS. A baseball pitcher uses Bernoulli’s principle to throw a curve ball. The ball is launched with a strong spin so that one side of the ball has a greater velocity toward the batter than the other side. That means that on one side the air is moving faster relative to the ball and therefore exerts a lower pressure. On the other side the air is moving slower, exerting a higher pressure. The ball is pushed toward the lower-pressure side, causing its path to curve.