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Air Streams

Introduction
Public summary: 

Can you make something float on thin air? Find out how to levitate ping pong balls and why planes can fly in this entertaining experiment.

Balancing balls on upward streams of air, and looking at aerodynamics.
Useful information
Kit List: 

Small scale stuff:
- Bendy straws (consumable), must have reasonably wide holes.
- Ping-pong balls
Large scale stuff:
- Big Blower (too big for box, needs 2 people to lift)
- Stream stabiliser for the big blower (a hollow box open on one end and with a grid on the opposite end)
- 'Small' blower (in box)
- 2 x variable power switches for blowers.
- beach balls
- Screwdriver with haztape streamer (for showing streamlines)
- Paper
Wind tunnel stuff:
- Aeroplane wing

Packing Away: 

Everything except the big blower packs into one big blue box. This can be quite hard to do, the trick is to put the small blower and wooden stand in first, then fit everything else around it.
WARNING: Box is heavy to lift, the big blower is even heavier, don't try to lift it on your own.

Frequency of use: 
4
Explanation
Explanation: 

In a nutshell

Show how flowing air acts on object in or next to it.

How to set up the experiment

Note: The big blower is not necessary but makes a nice show. It may cause the circuit breakers to go off, so check first so that you don't cause a power outage a the beginning of the session.

Place the blowers on the ground. Make sure they are not in anyone's way. Put the stream stabiliser on top of the big blower. Plug the cables to the power switches and to the grid.

Note: The switch for the big blower gives full power for a few seconds after switched on. Afterwards you will be able to regulate it.

Blow up the beach balls and have the straws pingpong balls and aeroplane wing handy.

How to proceed

This is a suggestion on how to proceed with the demonstration. The demonstrator should encourage the kids to try and figure parts of explanation out by themselves by asking the right questions, so that they stay focused. There's a lot in here: you may not explain all of it!

Straws and pingpong balls

Take a straw and bend it in right angle. You can start by asking the kids if they can think of a way to balance the ball on top of the short end (not holding it by hand). Two ways are quite good: sucking and blowing. (Simple balancing it is possible but very hard.)

If you suck the air from under the ball, the air pressure from the outside will hold it in place. You can tell them about the air pressure and that we are constantly pressed everywhere and that suction occurs when we reduce the pressure in an area (eg the inside of the straw). (The concept might be hard to grasp so consider the age of the children.) They might already have some knowledge of it if they have seen the Vacuums experiment.

If you blow into the long end and hold the short end upwards, you are able to balance a pingpong ball on the stream. (The straw end needs to be vertical and you have to blow quite hard.) It requires a bit of practice. Let the kids have a go at it. Give each a new straw and dispose of them afterwards to avoid spread of infection. Demonstrating what goes on is easier with the blowers. It's the same on a larger scale.

A bit of theory - what are forces

Hold a ball in mid air and ask them what is going to happen if you let go. Show them that they are right. Now ask them why the ball falls down. You should get to gravitational force. Explain what a force is. That it is what makes things move, for example pushing, pulling, gravity, magnetic force, friction. Explain that if something isn't moving, the forces must be balanced (they are pulled down by gravity but pushed up by the ground).
Tell them about Newton's third law - action and reaction. If you push or pull something, you get pushed/pulled yourself. You can demonstrate by letting them push/pull your hand.

Blowers and beach balls

Preferably use the big blower for this. If you place the ball in the air stream it will balance on it and not fall off. You can demosnstrate that it always balances at the same height. If you hold the ball at the edge of the stream you can feel how it is pulled in. You can let the kids to feel it too. You can use the screwdriver with haztape to demonstrate the direction of the stream.

Explaining why the ball doesn't fall directly down is easy and the kids might tell you that on their own. The tricky part is why is it pulled to the middle of the airstream. There are two ways to go about the explanation. The first is using Newton's third law. When you look at the airstream around the ball when it is at the edge of the stream, you will see that it is bend around the ball. After passing the ball, the stream is not strictly vertical (it is going a bit sideways). That means that the ball effectively pulls the passing air towards itself. The equal and opposite reaction on the ball then is that it is pulled inside. The other way of explaining it is using the Bernoulli's principle - the fact that the pressure is lower where the air has greater speed. (You can demonstrate it by blowing between two parallel sheets of paper hanging vertically next to each other.) So there is atmospheric pressure on one side of the ball and lower on the side of the airstream. The difference in pressure results in a force towards the stream.


A bit on stability

By the time you're reading this we should have a few beach balls of various sizes in the box, if not shout at Zephyr...

Using these we can do a brief discussion on stability of systems and how they will tend to rearrange themselves in the most stable form.

If we put two balls of different sizes into the air stream at once, the most stable configuration is the smallest on the bottom and the largest on top. This is because each ball effectively blocks some of the air hitting it from getting to the ball above (though by no means all, as seen with the bigger at the bottom) and hence there is less upward force on the higher ball.

Thus with the smaller on the bottom, enough air still reaches the second ball to hold it up, whereas the other way up little air makes it to the smaller, higher ball and it is likely to 'fall' off the side of the stream.

Why the quotation marks? Well, if you're lucky (/careful with the settings) the second ball rarely actually escapes the stream, instead the larger ball barges it's way past and then the smaller slots back into place under it. I.e. the system seeks out the most stable configuration.

That said, the smaller ball on top is still an equilibrium state, just less stable, and for more advanced audiences you can talk about the differences between equilibrium and stability.

Note, there is another effect with very small balls, like the ping pongs balls, where they are so light that the force they experience hugely outweighs the difference in amount of air hitting them, so make sure that the balls are of roughly comparable mass, or the lightest will just fly off!

Aeroplane wing

When you tilt the small blower so that the stream of air is horizontal, you can demonstrate the forces acting on the aeroplane wing. Children should be able to feel the air pushing the plane upwards when it is held horizontally. The wings are designed to maximise the upward thrust (which means they push down air significantly) while the backward thrust is minimised (we don’t want the wings to slow down the plane too much). Tilting the wing in the airstream results in greater upward force but also greater backwards force. Let the kids feel it but be careful so that the wing doesn’t fly off and hit them in the face.

Two possible explanations you can use:

One way to think about this is again with Newton's Third Law. The curvature of the wing means that the wing pushes the air blowing at it downwards, and so the air pushes the wing upwards. The size of the force depends on how fast the air is moving, and therefore how much air it is pushing down - you can show this by adjusting how hard the fan is blowing. This means that a plane has to travel VERY fast to get enough lift to keep it in the air (commercial airliners travel around 500mph).

Or using air pressure to explain: When the plane moves forward it pushes air out of the way, over and under the wing. As the top of the wing is curved more than the bottom of the wing, the air has to go further to get over the top and so it has to move faster. Bernoulli's principle says that the pressure is lowered when the air speeds up, so the air pressure above the wing is lower than the pressure below it, and the wing (and plane) is pulled upward (ie the same way things get sucked into a vacuum), opposing the gravity pulling it down.
[note: this doesn't explain why the air has to move faster over the wing - the air stream moving over the top of the wing has to move through a narrowing space due to the curvature, but with a constant volume per time (like water through a tube that then gets narrower).]

Bits from older explanation

*** OTHER THINGS TO TALK ABOUT ***

Have you ever noticed your ears going pop as your train goes through a tunnel?

For the train to move forward the air in front of it has to get out of the way, normally there is lots of space for it to do this, but in a tunnel there is just a little gap around the side, which the air has to squeeze through really fast.

This fast moving air sucks some of the air out of the train, which then sucks on your eardrums, making them go pop!

Why do you think they go pop again when you leave the tunnel?

The air isn't going as fast outside so the pressure goes up and the pop back inwards again.

You may notice the ball is spinning sometimes - think about where the air must be flowing to stop the ball falling off.

This happens when the straw is at an angle so the ball drops down a little and the air stream is faster over the top which is what is holding it up.

Try moving your hands across the blower, just above the grill. This will cause the ball to 'dance' as you are changing the air stream so the beach ball adjusts its position to remain in equilibrium.

Risk Assessment
Date risk assesment last checked: 
Wed, 28/12/2016
Risk assesment checked by: 
Andrei Ruskuc
Date risk assesment double checked: 
Wed, 25/01/2017
Risk assesment double-checked by: 
bakrill
Risk Assessment: 
DESCRIPTION Try to keep ping-pong ball suspended in air by blowing upwards alone and by blowing upwards through a straw. Keep beach ball and balloons in air stream of several different air conditioning generators. Model wing in air stream of air conditioning generator.
RISKS 1.Finger hazard in blower.

2.Risk of long hair getting sucked into intakes.

3.Heavy object.

4.Electric risk + tripping over wires.

5.Slip hazard from ping-pong balls.

6.Transmission of illnesses via saliva on straws.

7. Risk of child injuring mouth from falling over with a straw in his/her mouth. Also, children are liable to run across people's paths after escaped ping pong balls, which can lead to accidents

8. Model wing can be blown quite hard, e.g. out of a young child's hand. Straws may be blown towards face or eyes.
9. Risk of dizziness from blowing too hard
ACTION TO BE TAKEN TO MINIMISE RISKS

1.Have designed guards so it is not possible to get a finger of any size into the danger area.

2.Keep children with long hair (and animals if applicable) away from intakes.

3.Keep air conditioning generator on floor. Make sure that it is stable in the mount. Take care when moving it.

4.Tape wires to floor and lay sensibly (not across the middle of the room). If necesssary, attach something brightly coloured so that cables are clearly visible. Follow electrical parts RA.

5. Keep track of where the ping pong balls are and do not leave them on the floor.

6.Give every child a fresh straw.

7.Get the kids to sit down when playing with the ping pong balls so that they don't run around with the straws in their mouths and fall over. Encourage them not to chase balls. Do not allow young children to take straws away with them after experiment.
8. Make sure model wing is held securely - do not let small children hold unassisted when blower at full power. Discourage dropping of straws into air streams.
9. Warn before blowing into the straws not to blow too hard and to stop if dizzy.

ACTION TO BE TAKEN IN THE EVENT OF AN ACCIDENT 1.Turn off electricity.

2.Call first aider in event of injury.

8. Call first aider in event of injury.

9. Call first aider in event of injury.
This experiment contains mains electrical parts, see separate risk assessment.
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