Lift (force)

Air can push on things.

This push can move things up. This push is called lift.

When air flows around a wing, it pushes it. The wing pushes the air down. Then the air pushes the wing up.

Many things use this push. Birds and bugs use it to fly. Kites and planes use it too.

Some things use lift in water. Boats and ships can use it. Even some seeds use it to fly from trees.

It is a very cool way to move!

Lift is a push from a moving fluid. A fluid is something like air or water.

When air flows around an object, it exerts a force. Lift is the part of that force that moves the object up or sideways. It is not always up. It is always at a right angle to the flow.

One way to think about lift is through Newton's third law. This law says every action has an equal and opposite reaction. As air flows over a wing, the wing pushes the air down. Because of this, the air must push the wing up. This upward push is lift.

Many things use this force to move. Planes use wings to fly through the air. Birds, bats, and insects use it to glide. Even some tree seeds use lift to travel. In water, things like boats and hydrofoils use it too. If the fluid is air, we call it an aerodynamic force. If the fluid is water, we call it a hydrodynamic force.

Lift is a special force that happens when a fluid moves around an object. A fluid is something that can flow, like the air around us or the water in an ocean.

When this fluid flows past a solid shape, it pushes on that shape. Lift is the part of that push that moves at a right angle to the flow. Most people think of lift as an upward force that fights against gravity.

However, lift can actually act in any direction as long as it stays perpendicular to the flow. If the fluid is air, we call it an aerodynamic force. If the fluid is a liquid, we call it a hydrodynamic force.

There are different ways that lift can work depending on the situation. One way is called aerostatic lift, or buoyancy. This happens when the fluid inside an object is lighter than the fluid outside.

This is how big balloons, blimps, and even submarines work without needing to move. Another way is called planing lift. This occurs when only the bottom part of an object sits in a moving liquid. You can see this happening with motorboats, surfboards, and even water-skis. These objects use the moving water to stay on top.

Scientists use different ideas to explain how wings create lift. One famous idea uses Newton's third law of motion. This law says that every action has an equal and opposite reaction.

As air flows over a wing, the wing pushes the air downward. Because the wing pushes the air down, the air must push the wing up with the same amount of force. This upward reaction is what we call lift. Another way to look at it is through Bernoulli's principle. Both of these ways are correct and help us understand the complex way fluids move.

It is hard to explain lift perfectly because the math is very complex. Many people try to use simple models, but these models often leave things out.

For example, some people wrongly claim that air travels faster on top because the path is longer. NASA experts have noted that many textbooks actually use incorrect diagrams or explanations. Even describing how the air turns around a wing is a very hard job. A change in velocity in one direction can cause a change in a perpendicular direction in a fluid. This is much more complicated than how solid objects move.

We see the power of lift in many parts of our world. It is not just for airplanes with fixed wings.

Many other things use streamlined shapes to create lift, such as propellers, kites, and helicopter rotors. Even racing cars use wings to create lift in a different direction. In nature, birds, bats, and insects all use lift to fly through the sky. Even some seeds from trees use it to travel through the air. It is a rule of nature that connects machines, animals, and plants.

Lift is a fundamental force that occurs when a fluid flows around a solid object. In physics, a fluid is any substance that can flow, such as air or water.

When a fluid moves past an object, it exerts a force on that object's surface. Lift is specifically defined as the component of this force that acts perpendicular to the direction of the oncoming flow. This is different from drag, which is the component of the force that acts parallel to the flow. While we often think of lift as an upward force that opposes gravity, it can actually act in any direction as long as it remains perpendicular to the fluid flow.

There are different ways to categorize how lift is generated depending on the medium and the method. When the surrounding fluid is air, the force is called an aerodynamic force. If the fluid is a liquid, such as water, it is known as a hydrodynamic force.

Some lift does not require movement at all, which is called aerostatic lift or buoyancy. This happens when the fluid inside an object is lighter than the fluid surrounding it, a principle used by blimps, balloons, and submarines. Another type is planing lift, which occurs when only the lower portion of an object is immersed in a moving liquid. You can see this in action with motorboats, surfboards, and water-skis.

Understanding the mechanism of aerodynamic lift is a complex task in fluid mechanics. Scientists often use airfoils, which are streamlined shapes designed to generate more lift than drag.

The flow around an airfoil is difficult to describe because fluids can redistribute their mass while conserving momentum and energy. A change in velocity in one direction can cause a change in a perpendicular direction, which is not something that happens in solid mechanics. Because of this complexity, researchers use mathematical theories to represent the flow accurately. These theories require solving difficult equations to account for how the fluid moves around the shape.

There are two primary physical ways to explain how an airfoil generates lift. One approach uses Newton's Third Law of Motion, which states that every action has an equal and opposite reaction.

As air flows over an airfoil, the shape causes the air to be deflected downward. Because the wing exerts a downward force on the air, the air must exert an equal and opposite upward force on the wing. This reaction force is the lift. The upper surface of the wing actually contributes more to this downward turning of the air than the lower surface does. This process creates a downward velocity component in the air stream.

Another common approach is based on Bernoulli's principle, which relates the speed of a fluid to its pressure. While many textbooks present Bernoulli's formula, experts note that explaining lift using only this principle can be incomplete. A complete explanation must account for how pressure differences are sustained and how the air follows the curve of the wing. Some older, incorrect theories suggested that air must travel faster on the top of a wing because the path is longer. However, NASA and other researchers have clarified that this "equal transit-time" explanation is wrong. Both the Newtonian and Bernoulli approaches are considered valid ways to view the same physical phenomenon.

Lift is not limited to the wings of airplanes. Many different streamlined bodies use these principles to function.

Propellers, helicopter rotors, and kites all generate lift to move or stay aloft. Even racing cars use wings to create downforce, which is lift acting in a downward direction. In the water, maritime sails, ship rudders, and hydrofoils use hydrodynamic lift to navigate. Even the natural world relies on these forces. Birds, bats, and insects use aerodynamic lift to fly, and some tree seeds use it to travel through the air.

In summary, lift is a versatile force that connects many different systems in our world. It can be used to stay afloat through buoyancy, to skim across water through planing, or to fly through the air via aerodynamic deflection. Whether it is a massive submarine using aerostatic lift or a small insect using aerodynamic lift, the underlying physics remains the same. By understanding how fluids interact with solid shapes, we can design everything from wind turbines to advanced aircraft. The study of lift continues to be a vital part of science and engineering.