# Bernoulli's Principle

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## Introduction: Bernoulli's Principle

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"Look down at the veins tracing their way up your fingers. They are made of stardust. This is a truth rooted purely in science, one of the many that make the subject beautiful to me. However, it is a shame most science students will not realise this during the course of their studies. What could have been a passion for astrophysics becomes instead distaste for formulae of elliptical orbits students are unable to fully comprehend. Were I to change the way science is taught in institutes, I would encourage appreciation for the implicit over memorisation of the explicit, nurture understanding and curiosity for derivations over worship of formulae, and demonstrate practical applications to clarify ambiguous theory." - Sahr Jalil 2011

My friend Sahr, wrote this for our team's evaluation form to LUMS PSIFI 2012. The question was, "How would you change the way science is being taught in institutes?"

Science cannot be taught just based on theory, but it needs to be taught based on observation, something formula and rote-learning cannot teach. And this example, is perfect to reflect over this thesis.

## Step 1: Definition

The Bernoulli's Principle was a physics principle formulated by Daniel Bernoulli that an increase in the speed of a fluid produces a decrease in pressure and that a decrease in the speed of a fluid produces an increase in pressure. The principle states that the total energy of a moving fluid remains constant at all times. Therefore fluid pressure is inversely proportional to fluid velocity.

Practical Applications:

An Aeroplane relies on Bernoulli's Principle to generate lift on its wings.
A Helicopter uses the same method to generate lift on its wings.
A Race-car uses Bernoulli's principle to stay on the ground (down force).
An Insecticide Spray also uses Bernoulli's Principle to spread out the spray over a larger area.
A Bunsen Burner

Simple Demonstrations:

Envelope Experiment
Ping Pong Ball Experiments
Balloon Experiment
Paper Experiment
Boomerang

Lets first look over one practical application and then demonstrations and then more practical applications. :)

## Step 2: Practical Application : Airplane and Helicopter Wings

An airplane is designed in such a way so that it can be lifted easily while take off. The wings of the plane are designed in a streamlined shape. While taking off, air travels with a greater velocity over the wings than under the wings. The greater velocity creates a lower pressure area over the wings. A lower velocity creates a higher pressure under the wings. This pressure difference, makes the airplane climb higher and higher. The Pictures explain the same principle. Therefore Airplanes and helicopters use Bernoulli's Principle to climb.

## Step 3: Demonstrations : Paper Airplane

Materials Needed: Paper, Scissors

Everyday, someone designs a new paper airplane but not every plane is able to fly properly and go higher using Bernoulli's Principle. After bending a piece of paper into the respective folds, we are supposed to get a plane that it thicker from the front and thinner from the end. This helps to give the paper plane better lift.

Another Way of demonstrating this experiment is to make two paper airplanes of the same paper and structure and then cutting one from the ends as shown in the pictures.

Airplane 1 : No cutting.
Airplane 2 : Cutting

We notice that Airplane 2 actually lifts and then lands showing a parabolic curve. Airplane 1 just shows a downward movement.

## Step 4: Demonstrations : Blowing Over Paper

Materials Needed : Paper, Scissors

A stream of air over the paper creates a low pressure zone which lifts the paper due to the pressure difference: higher pressure under the paper and lower pressure over the paper. This levitates the paper in the air.

## Step 5: Demonstrations : Tissue Paper Hair Dryer or Leaf Blower

Materials Needed: Tissue Paper Roll, Blow Dryer, Metal Rod

Slip a thin rod through the hole in a toilet tissue paper role. Have one person hold the rod by placing a hand on each side of the toilet paper roll. Then switch on the blow dryer and watch the magic. This is Bernoulli's Principle which lets the tissue paper fly high up in the air without falling.

## Step 6: Demonstrations : Ping Pong Ball Experiment 1

Materials Needed: Ping Pong Ball, Blow Dryer

A ping pong ball can be "floated" on a stream of air. The air rushing around the ball creates a pressure low enough to lift and support the ball. Even when the ball is not exactly over the air source! As long as the low pressure spot is under the center of mass of the ball, it will stay "afloat". And the high pressure regions around the ball push the ball in if it tries to escape the low pressure zone.

We can repeat the same experiment using a balloon using a lower blow dryer power.

## Step 7: Demonstrations : Ping Pong Ball Experiment 2

Materials Needed : Ping Pong ball, Paper, Tape

In this experiment, we need to create two types of funnels. One will be a cylinder of the exact same radius of the ping pong ball itself and the other will be made in a cone shape. We now need to place the ping pong ball in both funnels and blow out from one end. The ping pong ball shoots out from the cylinder but not the cone shaped funnel.

When the ping pong ball is placed in the cone shaped funnel with the air blowing out, the ball won't fall out of the funnel. The rushing air creates an area of low pressure that holds the ball in place.

## Step 8: Demonstrations : Boomerang!

Most boomerangs are designed in a shape which helps them to stay in the air without falling and come back to the place from where thrown.  A boomerang has “arms” shaped like airplane wings which create lift. The boomerang is moving forward and the boomerang is spinning thus creating an uneven lift. The uneven lift created by the spinning boomerang makes it turn (like leaning on a bicycle)—and keep turning until it makes a circle and comes back to you.

## Step 9: Demonstrations : Envelope

Blowing air over the open end of an envelope causes the two sides to separate due to the lower pressure of the flowing air. This happens because air from the high pressure regions surrounding the envelope rushes towards the lower pressure area thus opening the envelope and temporarily inflating it.

## Step 10: Practical Applications : Race Car

A Race Car, in fact almost all cars are designed in a way to avoid lift and stick to the ground at all times. We wouldn't want cars just suddenly rising up due to high velocity. So keeping in mind the Bernoulli Principle scientists designed cars in a way completely opposite to that of an airplane. A race car employs Bernoulli's principle to keep its rear wheels on the ground at all times while travelling at high speeds. A race car's spoiler—shaped like an upside-down wing, with the curved surface at the bottom—produces a net downward force, thus keeping the car to the ground.

## Step 11: Practical Applications : Insecticide Spray

When the plunger is pushed in, the air flows at a high velocity through a nozzle. The flow of air at high velocity creates a region of low pressure above the metal tube. The higher pressure of the atmospheric air acts on the surface of the liquid insecticide causing it to rise up the metal tube. The insecticide leaves the top of the metal tube through the nozzle as a fine spray.

## Step 12: Practical Applications : Bunsen Burner

When the Bunsen Burner is connected to a gas supply, the gas flows at high velocity through a narrow passage in the burner, creating a region of low pressure. The outside air, which is at atmospheric pressure, is drawn in and mixes with the gas.
The mixture of gas and air enables the gas to burn completely to produce a clean, hot, and smokeless flame.

## Step 13: Ending

So, there we have it; Bernoulli's Principle. It is quite surprising that such a small and simple physics principle is used almost every second in our daily lives. I hope the videos helped. :)

Manish Kumar
Sahr Jalil

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## Tips

0

Neither small nor simple. The iteration of Bernoulli's equation I used working on nuclear powered propulsion systems for the Navy has 26 terms. Try again.

## Questions

Hi, my name is Integza and i have a education channel on youtube.

I have a video explaining the Bernoulli's principle, is very graphical and easy to understand because doesn't use much technical terminology:

As I commented under your Bernoulli video, you work too hard to justify Bernoulli. The pressure is higher in the large section because there is a constriction where the diameter reduces. The pressure in the smaller section is lower because it is free to flow away from the higher pressure region. If you view the smaller section as simply a hole in the pressurized large section (view it as a pressurized tank), the fluid easily escapes out this "hole" to the lower pressure region. It is pressure differences that cause accelerations in a fluid. A higher pressure region represents a force (F) that pushes (accelerates A) the mass (M) of air toward a lower pressure region. F=MA.If you have a gradual change in diameter, the momentum description is not valid.

--

Cheers

The majority of these demos are classical misinterpretations of the applicable science as well as Bernoulli's actual Principle. Some of the other comments bring-in many things including math which DOES NOT explain the physics/science.

The true physics is not difficult to understand if you can get all these misconceptions out of your mind.

Speed of a fluid DOES NOT create a lower pressure.

Speed past a surface (or visa versa) does not create a lower pressure.

Bernoulli mentions neither of these.

If a stream of fast air did have a lower pressure than surrounding air, then, the fast stream would be squeezed narrower and narrower by the higher pressure around it as long as it moved and this simply does not happen.

Pressure at a point in a fluid acts in all directions; can't violate that. Therefore, the pressure in the stream (at its outer boundary) pushes out with the same pressure as the atmospheric pressure outside it pushes inward.

Paper airplanes (flat balsa wings and Wright Bros wings) have no curved, 'LONGER' upper surface, so this well known bad explanation has a serious problem.

The lifting paper and tissue paper/leaf blower are a curved airflow followed by entrainment (from viscosity).

This floating ball explanation is messed up. If the pressure was lower UNDER the ball it would not be held up. Then, It is centered by curved airflow.

The atomizer (insectide spray) is due to a curved airflow (commonly called "turning" of the air) over the vertical tube's end.

The Bunsen burner is due to entrainment (viscosity).

The envelope opens because you blow air into it, thus increasing the internal pressure.

Air has mass and a force is required to accelerate mass (even air). That force is pressure (difference) and pressure alone.

A CURVED AIRFLOW requires a force because this is an acceleration. This force can only be a pressure difference. That pressure difference is from the relative motion of fluid and object.

Pressure differences are created DIRECTLY by the relative motion between fluid and object. [no Bernoulli "fast" air, no half venturi pinching, no other false science]. Walk through water and feel how you must push some water around! You create a higher pressure in front of you by pushing on the water (and lower pressure behind by moving away from water). THAT'S WHAT CREATES THE PRESSURE CHANGES.

That pressure difference (cause) accelerates water (effect) around you. A higher pressure region accelerates (pushes) air toward any (and all) lower pressure regions. The same thing applies to wings and air: except, to our advantage, the higher pressure is under the wing; lower above.

These very same pressure differences then cause all of the the fluid accelerations we see around a wing. Anything else is bad science and violates Newton.

This simple concept explains all the air flows around the wing (including the up-wash and down-wash).

The air above and below a wing are unrelated and Bernoulli can not be used to compare velocities and pressures. Therefore, saying the upper air is lower pressure because of the difference in speed to the lower air is completely false science and terrible math. Bernoulli deals with accelerations and pressures that occur along a single path (streamline).

Again: A higher pressure region accelerates (pushes) air toward any (and all) near-by lower pressure regions.

Also, it is the path the air is forced to take by the wing that is what you should focus in, not simply the wing's shape.

Finally, (once you fully understand it, it'll be no surprise that) from the siationary air's frame of reference, the fastest moving air around a wing is UNDER it ! This is easily shown on real wings. This blows the "fast-air-has-lower-pressure-than-some-nearby-slower-air" out of the water. That is a serious and long standing misunderstanding by amateurs, but not those in the field.

Don't believe me.

For a better understanding of the true physics, see these authoritative references:

Krzysztof Fidkowski How Planes Fly

David Anderson:

Babinsky;

McLean (though he gets rather heavy later in the video):

Regards, Steve Noskowicz

challengerillinois.org

The discussion between valhals_end and lemonie/eplunket was AWESOME!!! way to go Valhalas_end.

Thank you, this really helped with my homework. 8/8!

Very Informative! (y)

The curves :)

If  airplanes relied upon Bernoulli's Principle, they wouldn't be able to fly upside-down would they?

L

If they only flew in an incompressible (and ideal) flow regime, then technically yes, depending on the airfoil shape - there are symmetric wing designs that allow low-speed airfoils to fly upside down (but you can't neglect the fuselage and tail influences on aerodynamics when you extend from infinite-wing studies - see Prandtl Lifting Line theorems on airfoils - to fully 3D flow over airplanes). Most wings see compressible flow, though (if not fully spread across the wing, like in high transonic or supersonic flow, then in pockets of influence, which can be forced to occur on special airfoil designs down to Mach 0.3), where Bernouilli's Principle is superseded by compressible gas dynamics.

Most of these practical examples would operate at high enough speeds so compressibility would be an issue, but for simple understanding, Bernouilli's Principle can be applied.