What is Lift?

If you've seen an airplane in flight, you may have wondered how it stays in the air. Isaac Newton discovered gravity is a force that pulls objects toward the earth. This means that Earth's gravity pulls on the airplane! If that's true, how does the plane stay in the air?

In the early 1700s, Newton proposed that for every force, there is an equal and opposite force. In other words, for an airplane to stay in level flight, there must be an equal and opposite force to gravity as it acts on the plane. Lift is the opposing force that allows an airplane to get off the ground and stay in the air.

Where does lift come from?

Lift is produced by a pressure difference. Daniel Bernoulli proposed that changes in air speed are related to changes in air pressure. The slower the air moves, the greater the air pressure. Air pressure is the force air molecules exert on one another. This force changes as air molecules move through the atmosphere.

Ted Tune is performing an experiment that shows Bernoulli's Principle at work. He is using a hair dryer to float a Ping-Pong ball. If he tilts the hair dryer, the Ping-Pong ball still floats. If he tilts it too far, however, the ball will fall because the lift holding it in the air is not great enough to overcome gravity.

Why did the Ping-Pong ball stay in the air?

First, let's consider the principles established by Bernoulli and Newton.

1. Air exerts pressure in every direction at all times. Bernoulli's Principle says that the slower the air moves, the greater the air pressure. Therefore, we can say that still air exerts more pressure around the Ping-Pong ball than moving air. This pressure difference is what keeps the ball floating even when the hair dryer is tilted.
2. According to Newton's laws, when one object exerts a force on another, the second object must exert an equal and opposite force on the first. The force of gravity pulls down the Ping-Pong ball while air moving upward creates lift. When both forces are equal, the Ping-Pong ball stays at roughly the same location in the air stream.

Did You Know? In the 1700s, Daniel Bernoulli, a Swiss mathematician and physicist, focused his research on aerodynamics. The study of aerodynamics deals with the motion of air and the forces acting on bodies moving through the air.

How does the shape of a wing affect lift?

If we slice the wing of a typical commercial airplane and look at its cross-section, its shape looks like a stretched-out water droplet. This aerodynamic shape is referred to as an airfoil.

Usually, the position of the airfoil in flight is set at an angle so that air first hits the wing's front edge and bottom. This causes the air stream to split. The air above the wing travels faster over a curved surface than the air below the wing. The faster air has a lower pressure than the slower air, and it is this pressure difference that generates lift.