Levitating on an invisible stream of air, a beach ball seems to defy gravity. If you try to pull the ball out, you can feel a force pulling it back in—the same force that keeps an airplane in flight.
When the ball is suspended in the air stream, the air flowing upward hits the bottom of the ball and slows down, generating a region of higher pressure. The high-pressure region of air under the ball holds the ball up against the pull of gravity.
When you pull the ball partially out of the air stream, air curves around the ball and flows outward. This outward-flowing air exerts an inward force on the ball, in keeping with Newton’s Third Law: For every action, there is an equal and opposite reaction. Similarly, the downward flow of air beneath a helicopter exerts an upward force on the blades of the helicopter.
You can also explain the ball’s behavior in terms of Bernoulli’s principle: When the speed of a fluid increases, the pressure in that fluid decreases. As you pull the ball out of the air stream, air flows faster over only one side of the ball. The difference in pressure between the still air and the moving air pushes the ball back into the center of the air stream.
Airplanes, too, fly by deflecting air. A flat wing tipped into the wind forces air downward. The reaction to the downward force of the wing on the air is the upward force of the air on the wing. You can feel this lifting force if you hold your hand out the window of a moving car and tip your hand so that it forces the air downward.
A wing that’s curved on top will fly even if it isn’t tipped, thanks to what’s called the Coandă effect, the tendency of a fluid stream to adhere to a surface it passes over. Air passing over the curved wing top “sticks” and follows the arc of the wing, ultimately getting deflected downward. This downward deflection of air produces an upward force on the wing.
The Coandă effect also makes possible the curve balls of baseball and the banana kicks of soccer.