Bernoulli's Principle: Definition and Examples

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  • 0:01 Bernoulli's Principle
  • 2:05 Applications of…
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Understanding how a moving fluid's speed and pressure change as it flows along is not only important for building airplanes but also for backyard fun. In this video lesson you'll learn about Bernoulli's principle and identify some practical applications of this concept.

Bernoulli's Principle

When I was a kid, one way that I could torment my siblings was with the garden hose. This simple piece of equipment provided hours of fun for me because I could use it to spray them and soak them with water. This couldn't be achieved by simply holding the hose out toward them, though, because the water would only run out of the end and onto the ground. I had to use my thumb to block a portion of the hose's opening to make the water come out faster.

At such a young age, I had no idea who Daniel Bernoulli was or why partially blocking the end of the hose made the water come out faster. All that mattered to me was that I had a powerful weapon in a regular, everyday piece of garden equipment.

I no longer spray my siblings with a garden hose, but I do now know the physics behind why this was possible (and so much fun!). The reason the water spray increased when the hose opening was blocked is because a fluid increases speed when it flows through a narrower space. The water coming out of the hose traveled faster when part of the opening was blocked because it was flowing through a narrower space than when the entire end of the hose was open.

You can see same thing happen in rivers as they change width. Water running through the wider regions travels more slowly but speeds up as it passes through the narrower parts. But what you may not believe is that the pressure in these narrower regions decreases within the fluid as the water speeds up. Likewise, the pressure in the fluid increases as the water slows down in the wider regions.

This discovery was made by Swiss scientist Daniel Bernoulli and is called Bernoulli's principle. Bernoulli studied fluids in pipes and found that where the speed of a fluid increases, the internal pressure in the fluid decreases.

This is not an easy concept to grasp. In fact, you may be thinking that if the water is in a tighter space, the pressure should increase. Well, it does, but not the pressure within the fluid. The pressure increase is experienced by whatever is surrounding the fluid. In fact, it's this change in pressure that actually causes the fluid to change speed, not the other way around.

Applications of Bernoulli's Principle

It might help to think of a traveling fluid in terms of streamlines. These are imaginary lines that represent the path of fluid particles. Streamlines are far apart when the area surrounding the fluid is wide. But when the area becomes narrow, the streamlines are pushed together, decreasing the pressure in the fluid and increasing its speed.

We can apply the idea of fluid streamlines to all sorts of situations. Since both liquids and gases are fluids, we can apply Bernoulli's principle to things like air as well as water.

Airplane wings provide a great example of this principle in action. Airplane wings are designed so that air will flow faster over the top of the wing than underneath it. The top of the wing has a greater curve than the bottom, and this curve crowds the streamlines together. Since the streamlines are closer together, there is less pressure in the fluid (the air) above the wing than below it. Since the pressure below the wing is greater, it creates an upward lift toward the area of lower pressure, pushing upward on the bottom of the wing.

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