A 400,000 Pound Machine Flies Through the Sky. Nobody Finds That Strange.
Every single day, hundreds of thousands of people board enormous metal machines and trust them to stay in the air for hours at a time.
A Boeing 747 weighs up to 400,000 pounds. It flies six miles above the Earth at close to the speed of sound. And almost nobody on board is thinking about why it does not fall.
That is exactly the kind of question Caroline asks in Why Don’t Spinning Tops Fall? by Charles DeLisi. On her way to the airport, she turns to her mother and asks: “Why do planes not fall?”
The answer involves two beautiful ideas working together. One is Newton’s Third Law. The other is the Bernoulli principle.
Let us break both down simply.
What Is the Bernoulli Principle Simple Version?
The Bernoulli principle states that when a fluid moves faster, its pressure drops.
Air is a fluid. When air speeds up, the pressure it exerts on nearby surfaces goes down. When air slows down, the pressure goes up.
That single idea, faster air means lower pressure, is at the heart of how wings generate lift.
Here is how it works on a wing.
A wing is not flat. It is shaped so that the top surface curves upward while the bottom surface is flatter. This shape is called an airfoil.
When a plane moves forward, air splits at the front edge of the wing. Some air goes over the top. Some goes under the bottom.
The air going over the curved top surface has to travel a longer path and moves faster. The air going under the flatter bottom surface moves slower.
Faster air on top means lower pressure on top. Slower air on bottom means higher pressure on bottom.
That pressure difference pushes the wing upward. That upward push is called lift.
That is the Bernoulli principle at work.
But Wait. Newton Matters Too.
The Bernoulli principle is not the whole story. Newton’s Third Law also plays a major role, and understanding both gives you the full picture.
Newton’s Third Law says: for every action, there is an equal and opposite reaction.
A wing is tilted at a slight angle as it moves through the air. This angle is called the angle of attack. Because of this tilt, the wing pushes air downward as the plane moves forward.
If the wing pushes air down, the air pushes the wing up. That is Newton’s Third Law producing lift directly.
At lower angles of attack, the Bernoulli pressure difference does most of the work. At higher angles, the Newtonian push becomes more important. In reality, both effects are always present and working together.
Caroline’s mother explains it this way during their drive to the airport. She asks Caroline to stick her hand out of the car window and tilt it at different angles. Caroline immediately feels the upward push when her palm tilts forward. She can feel Newton’s Third Law with her own hand.
Why the Shape of the Wing Matters
The airfoil shape is not accidental. It is the result of careful engineering based on fluid dynamics.
Daniel Bernoulli, the Swiss mathematician and physicist who gave us the Bernoulli principle, published his work on fluid dynamics in 1738. He had no idea it would eventually help humans build flying machines. That technology would not exist for another 170 years.
This is one of the most important lessons in the history of science. Mathematical discoveries made out of pure curiosity, with no practical purpose in mind, often become the foundation for technologies nobody could have predicted.
Bernoulli came from one of the most remarkable families in scientific history. He had no fewer than eight close relatives who made major contributions to mathematics and physics. His own PhD was not even in mathematics. It was in anatomy, a choice forced on him by his father.
Yet his work on fluid dynamics now helps keep hundreds of millions of people in the air every year.
Why Speed Matters So Much
The amount of lift a wing generates depends on several things: the shape of the wing, the angle of attack, and the speed of the plane.
Speed matters enormously, and the relationship is not simple. Lift increases with the square of velocity. If a plane doubles its speed, the lift it generates goes up by a factor of four.
This is why planes need to reach a certain speed before they can take off. Below that speed, the wing cannot generate enough lift to overcome the weight of the aircraft.
It is also why planes use a steeper angle of attack during takeoff and landing, when they are moving more slowly. The steeper angle compensates for the lower speed by pushing more air downward and generating more Newtonian lift.
At cruising altitude, planes reduce their angle of attack and rely more on speed to maintain lift efficiently.
What Happens If the Angle Gets Too High?
There is a limit.
If the angle of attack increases beyond roughly 15 to 20 degrees, something goes wrong. The airflow over the top of the wing becomes turbulent. Instead of flowing smoothly over the curved surface, it breaks apart and separates.
When that happens, the pressure difference collapses. Lift drops suddenly and dramatically.
This is called a stall. It has nothing to do with the engine. A plane can stall even at full power if the angle of attack is too high.
Pilots train extensively to recognize and recover from stalls. Modern aircraft also have sensors and warning systems specifically designed to prevent them.
Birds Use the Same Physics
Here is something worth stopping to appreciate.
Birds figured all of this out through evolution, hundreds of millions of years before humans worked out the mathematics.
A bird’s wing is also an airfoil. Birds adjust their angle of attack constantly as they fly, modulating lift with extraordinary precision. They do this instinctively, without equations.
The difference is scale and purpose. A bird weighs a few pounds and optimizes for energy efficiency and maneuverability. A plane weighs hundreds of thousands of pounds and optimizes for speed and endurance over long distances.
Same physics. Very different engineering solutions.
This is what Charles DeLisi calls biological engineering in Why Don’t Spinning Tops Fall? Nature is the greatest engineer of all. And understanding what nature figured out long ago continues to inspire human design.
The Bernoulli Principle in Everyday Life
Once you understand the Bernoulli principle, you start seeing it everywhere.
It is why a shower curtain blows inward when you run the shower. The moving air created by the water drops faster on the inside of the curtain, lowering the pressure. The higher pressure on the outside pushes the curtain in.
It is why a curveball curves in baseball. The spinning ball drags air around with it, creating faster airflow on one side and slower airflow on the other. The pressure difference pushes the ball sideways.
It is why carburetors in older engines worked. Fast-moving air through a narrow tube lowered the pressure, drawing fuel up into the airstream.
The same principle that keeps a 400,000 pound plane in the air also moves your shower curtain. Physics does not change based on the scale.
Caroline’s Question. Your Curiosity.
When Caroline sticks her hand out of the car window and feels the air push up against her palm, something clicks. The physics becomes real. It stops being abstract and becomes something she can feel.
That is the goal of Why Don’t Spinning Tops Fall? Not to give you facts to memorize, but to give you a way of seeing the world that makes the ordinary things around you suddenly fascinating.
Every plane you see in the sky is a live demonstration of Bernoulli and Newton working together. Every bird is a biological version of the same experiment.
The physics has been there all along. You just needed someone to point it out.
If you enjoyed this, you will also like Why Do Spinning Tops Not Fall? Physics Explained Simply, which explores how the same rotational physics keeps a spinning top upright.
Explore More Everyday Science
Why Don’t Spinning Tops Fall? covers flight, spinning tops, sound and music, light and color, AI, climate change, and much more through warm story-driven conversations between Caroline and her family.
Written by Charles DeLisi, Metcalf Professor of Science and Engineering at Boston University and a pioneer of the Human Genome Project.
Explore the science book for curious teenagers that started it all, or visit the Shop to get your copy today. Also available on Amazon.
Frequently Asked Questions
Q1. What is the Bernoulli principle in simple terms?
The Bernoulli principle says that faster moving air has lower pressure than slower moving air. On an airplane wing, air moves faster over the curved top surface, creating lower pressure there. The higher pressure underneath pushes the wing upward. That upward force is called lift.
Q2. Is the Bernoulli principle the only reason planes fly?
No. Newton’s Third Law also plays an important role. The wing is tilted at an angle that pushes air downward as the plane moves forward. By Newton’s Third Law, the air pushes back upward on the wing. Both effects, the Bernoulli pressure difference and the Newtonian reaction force, work together to keep the plane in the air.
Q3. Why do planes need to go fast before takeoff?
Lift increases with the square of velocity. A plane needs to reach a minimum speed before its wings can generate enough lift to overcome the aircraft’s weight. Below that speed, no amount of wing shape or angle can produce sufficient lift for flight.
Q4. What is a stall in aviation?
A stall happens when the angle of attack becomes too steep, usually beyond 15 to 20 degrees. The airflow over the top of the wing breaks apart instead of flowing smoothly. The pressure difference collapses, and lift drops sharply. Modern aircraft have warning systems to help pilots avoid stalls.
Q5. Do birds use the Bernoulli principle too?
Yes. A bird’s wing is also an airfoil shape, and the same pressure differences that generate lift on an airplane also act on bird wings. Birds adjust their angle of attack constantly and instinctively. Evolution produced this solution hundreds of millions of years before humans worked out the mathematics behind it.
