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== Age ==
{| class="wikitable" style="color:black; background-color:#ddd; margin-left: auto; float:right"
| [[Physics]]:
| Pressure, Bernoulli's Principle
|-
| Grade Range:
| [[Elementary School]], [[Middle School]], [[High School]]
|-
| Format:
| [[Hands-on]], [[Stage]]
|}


Elementary School, Middle School, High School
This is one of our most commonly performed demonstrations. It is easy to perform, easy to understand, and the topic matter is easily adjusted for all grades and age levels.
 
== Format ==
 
Stage Show, Hands-on


== Materials ==
== Materials ==


    Blower
* Shop Vacuum/Blower with Hose
    Blower hose
* Ribbon Wand
    Electrical Extension cord
* Beach Ball
    Beach Ball
* Round-bottom Plastic Bottle
    2-liter pop bottle, empty (with rounded bottom)
* Optional: Roll of Toilet Paper
    2-liter pop bottle, with some water (with rounded bottom)
    Roll of one-ply toilet paper
    Wooden Dowel (with fringe on one end)


== Safety Precautions ==
== Safety Precautions ==


Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience. Make sure objects are not blown into the audience.
Please read the Physical Demonstration section of the [[Demonstration Safety]] page before performing this demonstration.
 
== Preparation ==
 
Obtain all the materials listed above. Attach the blower hose to the air output of the blower. Ensure the blower has power and is operating correctly. Inflate the beach ball. Have on hand the two 2-liter pop bottles (one w/ water one w/o water). Have on hand the wooden dowel and toilet paper.


== Demonstration ==
== Demonstration ==


Part 1: Turn the blower on low power and while holding the hose at a sharp upward angle, find the position at which the beach ball will float. The wooden dowel can be used to show the moving air as the fringe will point in the direction of air flow. Notice the moving air on top of the ball, but no moving air under it. The dowel can also be used to show that there is nothing holding the ball in the air (ie. hidden strings, etc.)
Preparation: Blow up the beach ball. Make sure the hose is in the blower side, and plug it in. Make sure that there is a small amount of water in the soda bottle.
 
Part 2: Continue to demonstrate the principle as demonstrated in part 1, but this time use the empty 2-liter bottle.
 
Part 3: Continue to demonstrate the principle as demonstrated in part 1, but this time use the 2-liter bottle with water in it. At this time it also works best to have the blower on high power.
 
Part 4: Place the toilet paper roll on the wooden dowel and hold the toilet paper in front of the blower (on high power). The toilet paper will begin to unravel and fly off the roll. This can be directed into the audience if desired.
 
== What to Say ==
 
Part 1: Ask the kids if they think you can make the beach ball float in the air. Allow them to answer, then continue on to demonstration part 1. Explain that the air is moving faster on the top of the ball then on the bottom of the ball and that this means there is less pressure on the top and more pressure on the bottom. This causes lift and the ball stays aloft. Be sure to show the moving air on the top versus the still air on the bottom by using the fringe end of the wooden dowel.
 
Part 2: Start demonstration part 2. You can re-iterate the general concepts that you described in part 1, again showing the moving air versus the non-moving air. Again, explain that the fast moving air creates low pressure.
 
Part 3: Start demonstration part 3. Ask the kids if they think you can make a 2-liter pop bottle float if it has water in it and is heavy! Allow the kids to answer. Show the kids that it is in fact possible to make the water weighted bottle float. Remember to use the higher blower setting. You can again explain what is happening (high velocity = low pressure = lift). Be excited that the lift is strong enough to hold the bottle up!!!!


Part 4: Start demonstration part 4. Tell the kids that you are going to do something fun with Bernoulli's Principle. How about making a HUGE streamer of toilet paper!!!! Start by blowing the streamer to the side of the stage and then you can slowly move the paper so it is directed over the audience;. if desired. The paper is light and harmless so there is no inherent danger in this demo.
# Choose a volunteer from the audience and have them hold the ribbon wand. Explain to the audience that you will be making the beach ball fly by using the blower, but you want them to guess if the air will be blowing over or under the beach ball. Take a vote to see how many think the air should blow over or under.
# Turn on the blower on the "LOW" setting. Hold the hose at an angle slightly above 45 degrees, and lift the ball into the air stream. It will lift off into the air, and it will float! Have the volunteer hold the ribbons first under the ball, and see that the air isn't blowing underneath! Then, have them hold the ribbon wand over the ball to see the ribbons move, and show that the air blows over the ball!
# Let the volunteer return to their seat. Turn off the blower, and explain the demonstration.
# Call on a new volunteer, and have them hold the soda bottle in one hand. Turn on the blower on the "HIGH" setting. Holding the hose at about a 70 degree angle, have the volunteer place the rounded side of the bottle into the air stream. It should float in the air! After showing this for a minute, turn off the blower and explain.
# Optional: To end the show, call up one more volunteer. give them the ribbon wand, and have them hold it sideways, turned towards the audience. put the toilet paper roll on the wand, with the roll over side towards the audience. Turn on the blower on the "HIGH" setting, and aim above the roll a little above a 45 degree angle. The roll will unravel into the audience!


== Why This Works ==


An airplane's wing forces air to move very quickly over the top of it. Therefore the pressure of the air above the wing is lower than the air below the wing, by Bernoulli's principle - so airplanes can fly!
Bernoulli's Principle States: If you have an object inside of a fluid, then the object will move to the part of the fluid that exerts the least amount of pressure upon it. To understand what this means, we first look at the beach ball. Typically, the air is still around the ball, so the air exerts air pressure on all sides of it equally. Because it is equal on all sides, it will cancel out, meaning that the only thing left affecting the ball is gravity. If we use the blower, however, we now have the air above the ball no longer pushing down on it. instead, that air is now moving sideways. The air under the ball is not moving, though, and it is going to push the ball upward, against the force of gravity. The '''''lift''''', or upward force, is stronger than the force of gravity, so the ball stays suspended in the moving air.


Why It Is: This simplistic explanation of Bernoulli's Principle stems from Bernoulli's equation which states that the sum of all forms of energy in a fluid flowing along an enclosed path is the same at any two points in that path. This means that a change in pressure will cause a change in velocity. This is demonstrated by an airplane wing, where the change in pressure due to the curved surface changes the velocity at which the air flows because the sum of the velocity and pressure must be a constant.
When we are able to, we will include a few diagrams in this explanation to help visualize the science behind it.


1/2*Velocity2 + Pressure/Density = Constant
The bottle helps to get across how useful this knowledge is. Even though it has more mass to it, and therefore more weight, we can still get it to hover in the air by using this concept. The curve of the bottle's base allows it to float, much like the curve on a plane's wing will allow it to fly. A plane is able to fly thanks to utilization of this concept, along with a lot of important engineering to make the plane able to ascend, descend and turn in the air.


The above equation shows that as the pressure increases, the velocity must decrease in order to keep the sum truly a constant. It can also be said that as the velocity increases, the pressure must decrease in order to keep the sum at a constant. The above equation excludes the (gravity x height) term that should be included in the sum.
A more thorough explanation: In fluid dynamics, we can show that, as a fluid moves in a direction with a velocity v, that the amount of pressure that it exerts on any objects perpendicular to the motion will decrease. The equation is:


It can easily be seen that an airplane can generate some lift or upward force due to the higher pressure on the bottom of a wing versus a lower pressure on the top. This pressure is caused by the shape of the wing, forcing the increase in velocity and decrease in pressure. Please note that it is NOT caused by the air having to travel a longer distance over the curved side in the same time interval as the bottom of the wing. The reason it is traveling faster is actually due to conservation of mass (oddly enough). Note in the image below the two different air streams. The upper stream constricts as it flows over the airfoil. Due to conservation of mass laws, the air must increase its velocity over the airfoil and thus, by Bernoulli's Principle, exerts less pressure on the top of the wing. Conversely, the lower stream expands causing the air velocity to decrease and thus increasing the pressure.
'''1/2 v^2 + P/d = C''', where v=Velocity, P=Pressure, d=Density, and C=Constant, with units of (m/s)^2


== Why it is ==
By using this equation, we can see that as the velocity of the fluid increases, the Pressure needs to decrease in order for the constant to stay the same. This also helps us to understand one of the more tricky parts of this principle, which is that the reason why this happens is, surprisingly enough, due to the Conservation of Mass. As a plane's wing travels through the air, we are correct in that the air on top of the wing is moving faster than the air under the wing. However, this happens because the air the wing travels through has equal density throughout, and so the air traveling over the wing must re-meet the same part of the air traveling under the wing. Since the top of the wing has a longer surface, the air is forced to have a higher velocity, and therefore exert less pressure on the wing, so to make the full trip in time.


== Real Life Examples ==
== Additional Information ==


Airplanes, car spoilers, hydrofoils, inside carburetors
* This is not a typical "hands-on" demonstration, since the blower should not be operated by non-presenters. However, it does go over well at Science Fairs and Festivals!
* This demonstration is a part of the [[Pressure Show]].

Revision as of 17:27, 2 October 2015

Physics: Pressure, Bernoulli's Principle
Grade Range: Elementary School, Middle School, High School
Format: Hands-on, Stage

This is one of our most commonly performed demonstrations. It is easy to perform, easy to understand, and the topic matter is easily adjusted for all grades and age levels.

Materials

  • Shop Vacuum/Blower with Hose
  • Ribbon Wand
  • Beach Ball
  • Round-bottom Plastic Bottle
  • Optional: Roll of Toilet Paper

Safety Precautions

Please read the Physical Demonstration section of the Demonstration Safety page before performing this demonstration.

Demonstration

Preparation: Blow up the beach ball. Make sure the hose is in the blower side, and plug it in. Make sure that there is a small amount of water in the soda bottle.

  1. Choose a volunteer from the audience and have them hold the ribbon wand. Explain to the audience that you will be making the beach ball fly by using the blower, but you want them to guess if the air will be blowing over or under the beach ball. Take a vote to see how many think the air should blow over or under.
  2. Turn on the blower on the "LOW" setting. Hold the hose at an angle slightly above 45 degrees, and lift the ball into the air stream. It will lift off into the air, and it will float! Have the volunteer hold the ribbons first under the ball, and see that the air isn't blowing underneath! Then, have them hold the ribbon wand over the ball to see the ribbons move, and show that the air blows over the ball!
  3. Let the volunteer return to their seat. Turn off the blower, and explain the demonstration.
  4. Call on a new volunteer, and have them hold the soda bottle in one hand. Turn on the blower on the "HIGH" setting. Holding the hose at about a 70 degree angle, have the volunteer place the rounded side of the bottle into the air stream. It should float in the air! After showing this for a minute, turn off the blower and explain.
  5. Optional: To end the show, call up one more volunteer. give them the ribbon wand, and have them hold it sideways, turned towards the audience. put the toilet paper roll on the wand, with the roll over side towards the audience. Turn on the blower on the "HIGH" setting, and aim above the roll a little above a 45 degree angle. The roll will unravel into the audience!

Why This Works

Bernoulli's Principle States: If you have an object inside of a fluid, then the object will move to the part of the fluid that exerts the least amount of pressure upon it. To understand what this means, we first look at the beach ball. Typically, the air is still around the ball, so the air exerts air pressure on all sides of it equally. Because it is equal on all sides, it will cancel out, meaning that the only thing left affecting the ball is gravity. If we use the blower, however, we now have the air above the ball no longer pushing down on it. instead, that air is now moving sideways. The air under the ball is not moving, though, and it is going to push the ball upward, against the force of gravity. The lift, or upward force, is stronger than the force of gravity, so the ball stays suspended in the moving air.

When we are able to, we will include a few diagrams in this explanation to help visualize the science behind it.

The bottle helps to get across how useful this knowledge is. Even though it has more mass to it, and therefore more weight, we can still get it to hover in the air by using this concept. The curve of the bottle's base allows it to float, much like the curve on a plane's wing will allow it to fly. A plane is able to fly thanks to utilization of this concept, along with a lot of important engineering to make the plane able to ascend, descend and turn in the air.

A more thorough explanation: In fluid dynamics, we can show that, as a fluid moves in a direction with a velocity v, that the amount of pressure that it exerts on any objects perpendicular to the motion will decrease. The equation is:

1/2 v^2 + P/d = C, where v=Velocity, P=Pressure, d=Density, and C=Constant, with units of (m/s)^2

By using this equation, we can see that as the velocity of the fluid increases, the Pressure needs to decrease in order for the constant to stay the same. This also helps us to understand one of the more tricky parts of this principle, which is that the reason why this happens is, surprisingly enough, due to the Conservation of Mass. As a plane's wing travels through the air, we are correct in that the air on top of the wing is moving faster than the air under the wing. However, this happens because the air the wing travels through has equal density throughout, and so the air traveling over the wing must re-meet the same part of the air traveling under the wing. Since the top of the wing has a longer surface, the air is forced to have a higher velocity, and therefore exert less pressure on the wing, so to make the full trip in time.

Additional Information

  • This is not a typical "hands-on" demonstration, since the blower should not be operated by non-presenters. However, it does go over well at Science Fairs and Festivals!
  • This demonstration is a part of the Pressure Show.
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