According to the Bernoulli Principle, when a liquid or a gas flows more quickly, its pressure drops. This may seem backward, since it might seem like higher velocity would imply more pressure. In fact, though, when a liquid or gas flows more quickly, its energy shifts from static potential energy (static pressure) to dynamic kinetic energy (because of its higher velocity in a particular direction). The principle is named for Daniel Bernoulli, a Dutch-Swiss mathematician who lived in the 1700’s.
The Bernoulli Principle applies to many systems, but one of the most well-known is lift force on airplane wings. Because of how airplane wings are shaped, air moving across the top of the wing travels more quickly than the air moving across the bottom of the wing. This means the pressure on the top surface is less than the pressure on the bottom surface. Since pressure is force / unit surface area, whenever there’s a difference of flow rate across the top and bottom of the wing, there will be a resultant force. In turn, whenever there is a difference in forces acting in opposite directions on the wing, the resulting motion will be in the direction of the greater force, creating lift. Airplane wings have adjustable slats that let pilots change the wing’s shape and thereby adjust the lift force.
Another real-world example of the Bernoulli Principle is what happens when you constrict the flow of water through a garden hose. By closing off a portion of the flow cross-section, you force the water to speed up, such that the same volume of water can pass through a smaller area in the same amount of time. This increase in velocity decreases the pressure along the edges of the flow.
Another example of this principle is the slight “suction” effect that occurs when two large trucks pass by one another on the highway. As the space between the trucks narrows, the air flow between them increases speed, meaning that there’s more pressure on the other side of the trucks than between them. The effect is that they both have a resultant force towards one another and get “pushed” together slightly. You can also feel this same effect in a car as you pass by a large truck and are pulled slightly towards it.
Classroom Activity: Paper Lift
Students break into pairs. One student in the pair positions a piece of paper in front of his/her mouth and gently blows air across the top of it. The second student observes and then describes what he or she saw. Then the partners switch. Afterward, the class as a whole discusses the experience.
Classroom Activity or Teacher Demo: Ping Pong Pressure
Place two ping pong balls on a table. Using a straw, direct a flow of air between the two ping pong balls. Observe and describe what happens.
Koichi Wakata: In a fluid flow, when the flow gets higher, the pressure decreases. This fact is now well known as Bernoulli’s Principle. If you apply this principle, in a fluid flow, in a closed system, such as water flow in a tube and if you imagine the tube gets bigger and narrower, the pressure actually decreases when the speed gets higher. And when the speed of the water gets slower, the pressure actually increases. One of the examples I can give you is to think about the garden hose. If water is coming out from the garden hose at a maximum speed, if you still want to increase the speed of the water coming out of the garden hose, what you are going to do is you are going to put your thumb onto the exit of the hose. By doing that, the speed of the water increases and at the same time the pressure of the water decreases.
Frank DeWinne: We are placing these containers not so far from each other, about 5 to 10 centimeters. And then, Koichi is going to blow in the middle of those two containers. What do you expect will happen: because he blows in the middle, the containers are going to move away from each other, or, because he’s blowing in the middle, the containers are going to come together? Let’s have a look. Like you can very easily see here, the containers are drifting together. What is happening? Again, the air that Koichi is blowing through the containers has a higher velocity than the surrounding air. Because of that, the air traveling between the containers has a lower pressure because it’s moving at a higher speed. Because of that, the pressure on the outside is pressing the two containers together.
You can also observe this when you are driving on a highway and you get too close to a truck, for example, that is driving next to you. You will be drove towards the truck, and you need to steer away from the truck. So this is one of the examples, as well, of the Bernoulli Principle.
Koichi: Right now, Frank has a piece of paper that is curved. What he is going to do is, he is going to hold this piece of paper, and it’s curved, and he is going to blow on the top surface of the paper. What do you think this paper will do? Since the paper is curved like this, do you think the paper will come down, or come up? Let’s see what happens. As you see, when Frank blows air on the top side of the paper, the paper itself lifts up. This is again, one example of Bernoulli’s Principle. The air that goes on the top surface of the paper has a higher velocity that the air that is underneath the paper. Because of that difference in the velocity, the pressure of the upper surface is smaller than the lower surface. As a result, the paper is lifted up.
Frank: This is a principle, of course, that I have used a lot in my previous career, because I used to be a pilot, and this is how airplanes fly. If you look to the wing of an airplane, you see that it’s curved, always a little bit towards the bottom. The side on the top side is more curved and is longer than the bottom side. This means that the air traveling on the top of the wing will travel faster than underneath the wing, creating a lower pressure, and lifting the wing up. This is what we call lift, and because of this lift, we can travel and we can fly all around the world. Not only me, but you as well can travel wherever you want thanks to the Bernoulli Effect.
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