When you increase the pressure under a bubble, it will remain aloft. When you decrease the pressure over a bubble, it will remain aloft. Thus demonstrating Bernoulli’s theorem – for horizontal flow, as the speed of a fluid increases, the pressure it exerts decreases.
Derived by Daniel Bernoulli (see Bernoulli family), the principle explains the lift of an airplane in motion. As the speed of the plane increases, air flows faster over the curved top of the wing than underneath. The upward pressure exerted by the air under the wing is thus greater than the pressure exerted downward above the wing, resulting in a net upward force, or lift. Race cars use the principle to keep their wheels pressed to the ground as they accelerate. A race car’s spoiler—shaped like an upside-down wing, with the curved surface at the bottom—produces a net downward force.
p + qv2/2 + qgy = constant
- where p is the pressure, q is the density, v the velocity and y the height in a gravitational field of strength g, all measured at the same point. This quantity is then constant throughout the fluid.
The Bernoulli principle says that the faster the air flows, the less pressure it exerts. Think of an airplane as an example. As air hits the nose of an airplane, some of it has to go over the plane and some of it has to go under it. Regardless of whether the air goes over or under the plane, it arrives at the tail of the plane at the same instant. The air that goes over the plane has to travel farther in the same time period and therefore, it has to travel faster. Faster moving air exerts less pressure. Since there is greater pressure pushing up, the airplane will go up. Likewise will any object.
There are two approaches to making an object remain aloft: increase the pressure under it or decrease the pressure over it. Waving your hand over a bubble causes the air to move faster. Faster moving air exerts less pressure. The faster one waves, the higher the bubble will go.
- What do you think you can do to keep a bubble from hitting the ground without touching it with your hands or any other objects? Share your ideas.
- Use the tube to blow a bubble and then try to keep it from hitting the floor, but don’t touch the bubble.
- Do whatever you think you need to do to keep the bubble from touching the floor. Try different approaches to this task.
- What have you tried and what happened? Did you devise a method that works?
- Demonstrate Bernoulli’s principle by waving your hand back and forth over a bubble. * Use the index card by waving the index card above the bubble to practice Bernoulli’s principle.
- Write down how the experiment demonstrated Bernoulli’s Principal.
Materials Needed: Bubble solution; 1 container for solution; 1 empty TP tube (or can) per person; and 1 3″x5″ index card each. Prepare bubble solution: 1 cup dishwashing liquid (Dawn) in 1 gallon of water. If you are using cardboard tubes, put tape on the last two inches to make them last longer. If you are using tin cans, remove both ends of the cans.
- atmospheric pressure: This is also the pressure caused by air, but usually thought of as the normal value or standard value of 14.7 pounds per square inch. The atmospheric pressure at the place where we are on the face of the earth is usually less than this because we are above sea level, and hence have less “weight” pushing down on us. Again, used with the same activities as air pressure.
- pressure, low pressure, weight, force,