Lift is the force that holds an aircraft in the air. How is it generated?

It has been an extremely active debate among those who love flying and are involved in the field regarding which is best for describing how aircraft get the needed lift to fly? Two theories (Bernoulli's equation or Newton's laws and conservation of momentum) are the subject of an intense debate between scientists.

Who is correct?

Let’s first learn a little about both theories and do some simple experiments to illustrate them.

These experiments will help you understand better these two theories.Each experiment is easy to be replicated at home and it is designed to use everyday items that can usually be found in the home.

Even if all these experiments are very simple and funny they will introduce many advanced concepts and notions from physics like fluids, pressure, atmosphere and energy conservation. Also, they will help you understand some “tricks” used in sports (baseball, ping-pong) as well as some of the boomerangs and kites “secrets”.

Aerodynamic lift is a very complex phenomenon but by the end of this section you will have a pretty good image about it.


Sir Isaac Newton(1643-1727) 

Theory: Airplanes fly because wings deflect air downward so that in reaction the plane is forced upward.

Sir Isaac Newton is better known for his discovery of the law of universal gravity (every object in this universe attracts every other object with a force which is directly proportional to the product of their masses and inversely proportional to the square of distance between their centers).Another law of motion discovered by Newton states that for every action, there is an equal and opposite reaction(Newton's Third Law). As Newton’s laws suggests, the wing must change something of the air to get lift. To generate lift a wing must divert air down; lots of air. His theory is pretty and can be demonstrated easy.The kites are maybe the best example to demonstrate his theory in flight. 

Newtons Third Law of Motion is pretty intuitive and easy to understand. Watch this short video produced by NASA: 


Daniel Bernoulli(1700-1782) 

Theory:” Airplanes fly because the pressure above the wing is smaller than the pressure below the wing.”

Daniel Bernoulli, an eighteenth-century Swiss scientist, discovered that as the velocity of a fluid increases, its pressure decreases. Bernoulli's principle applies to any fluid, and since air is a fluid, it applies to air.

To understand how and why Bernoulli's Principle works, we can consider the following experiment (M.Mitchel).

“Take a room full of children, and ask each child to start running at top speed. Children will start bouncing off each other, and the walls, with impressive collisions (ouch!). Now take those same children out of the room, and ask them to run down the hall at top speed. Now they are all running together, and all collisions between children are much more gentle than before since they are all running in the same direction.

The children in both cases represent the atoms in the fluid, and the force of the collisions represents the pressure between those atoms. In the first case, when the speed of the group as a whole was zero, the jostling (or pressure) was high.

In the second case, when the speed of the group as a whole was large, the jostling (or pressure) was low.

There are two types of pressure involved in these experiment: static pressure and ram pressure.

The static pressure would correspond to the jostling felt by a teacher running along in the center of the children. That pressure would be slight. Ram pressure would correspond to the body blows experienced by an unsuspecting teacher standing in the path of the oncoming crowd (big ouch!).”

In the following video you will see a nice explanation of the Bernoulli principle:

The Bernoulli explanation of flight is not so intuitive as the Newton theory and one of the most difficult element to understand is how does the air on top of the wing know it has a "date" with the air under the wing?

It doesn't "know," but keep in mind that there is very fast moving wind behind it, urging it to keep moving! If the air on top of the wing moved slowly, there would be a huge buildup of air in front of the wing. If you put a rock in the bottom of a river, the river still flows at the same speed! But the water immediately above the rock has to speed up, lest it be "crushed" by the water behind it. This is why a great big wide river moves rather slowly, but as it passes through a narrow canyon it speeds up (rapids!). 

Another common question regarding the lift explained by the Bernoulli theory is why does air follow the curve of the wing? This phenomenon was explained by a Romanian scientist,Henri M. Coanda.It essentially states that "a fluid stream which comes in contact with a gently curved solid surface will tend to follow that surface". This can be easily observed by placing a spoon (or other curved surface) under a gently flowing faucet, and noticing how the water bends around the spoon. 

Here are some simple experiments to demonstrate Newton and Bernoulli theories 

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