The SFK Project: #6 Ping-Pong-Ball LevitationFollow article
What you need:
- A ping-pong-ball
- A plastic bottle half-filled with water
- A funnel (big enough to hold the ball)
- A straw
- If you have: compressed air and a blow-out gun, gaffa tape
What you will get at the end:
We will do an experiment where the ball levitates by blowing on it. In a second experiment, you will levitate the ball while it seals an open bottle with water inside.
How to perform the experiments:
Levitation by blowing
Put the ping-pong-ball inside the funnel. Press the ball against the wall of the funnel and turn the funnel upside down. When you would stop pressing the ball against the wall, it would drop onto the floor, right? But if you put the funnel’s tube into your mouth and suck on it, the suction does hold the ball in place. When you stop sucking the ball drops down. That seems to be quite natural, and you would have expected that, right?
But what happens when you blow into the funnel’s tube instead of sucking on it? The ball drops down – who would have thought. But now take a straw and enter it into the funnel. When you blow into the straw and take away your fingers from the ball, it magically levitates. That is somehow weird, isn’t it? The ping-pong-ball gets sucked into the funnel as long as you don’t run out of puff.
If you have compressed air and a blow-out gun at hand (who hasn’t…), you can use the compressed air instead of your lungs to blow inside the funnel’s tube. I taped the blow-out gun to the funnel's tube as you can see in the picture:
I used this combination for the next video. You will see us blowing air out of the funnel onto a hanky to proof that we blow air and do not suck in air.
Levitation by bottle
Better try this experiment over the kitchen sink. Things may get wet.
Take the bottle and fill it halfway with water. Put the ping-pong-ball on top of the bottleneck. Hold the bottle with one hand and press the ball against the bottleneck with the other hand. Turn everything upside down. Roll the ball a little bit to wet it all around while you carefully press the bottle a little bit with the other hand.
When you take your hand off the ball very slowly, you will witness a miracle: The ball sticks to the bottle, and the water stays inside the bottle. That’s what I call a funny bottle lid. I bet your parents will not believe that this will work. Try it over daddy’s favourite armchair… or maybe better not. There is always some water dripping down, and it also could go wrong. Ruining daddy’s armchair is not a good idea.
How it works:
Levitation by blowing
Please also read the explanations of FSK project #5 to learn about pressure and vacuum. When you suck on the funnel, you create a vacuum. The air above the ball has less pressure than the air below the ball. The ball is sucked in by this vacuum. But you could also say: The higher air pressure below the ball pushes the ball up into the funnel.
When a gas or a fluid is flowing fast, its pressure is less than when staying still. In the second part of our experiment, you blow air into the straw and above the ball. When the air passes the gap between the funnel’s wall and the ping-pong-ball, it streams very fast. This makes the air pressure less than the pressure of the resting air below the ball. See? It’s the same situation when you suck at the tube. So you get the same reaction: When the pressure above is less than the pressure below, the ball gets sucked into the funnel.
But what happens next? The ball gets sucked in and closes the opening fo the tube. Air can’t get through that opening any more. Because the air is no longer streaming fast, there is no longer low pressure above the ball. No suction – no levitation. The ball starts falling down. But as soon as it does, the air is streaming again, and it gets sucked in. This stop-go happens very fast and generates the humming sound.
Levitation by bottle
From the explanations of FSK project #5, you know that if something streams out of a bottle, it must be replaced by something else. When water streams out of our bottle, we get a low pressure inside the bottle. Usually, air would be sucked inside to replace the space of the water. The only way air could get into the bottle is through the bottleneck. It could – but in our experiment, it does not get into the bottle. Only a tiny portion has gone outside after we compressed the bottle. The rest stays inside. We have a low pressure inside the bottle on top of the ball and a higher pressure outside and below the ball. You have just learned in the funnel experiment what happens: The higher pressure outside is pressing the ball against the bottleneck and seals the bottle like a lid.
But why doesn’t air simply stream inside the bottle to fill the vacuum? There must be a tiny slit between ball and bottleneck where it could get in because water is dripping out there.
The reason is an extraordinary property of water: The small water particles on the surface of water cling together very strongly. Imagine they would be a crowd of people staying together and holding hands. If anyone wants to pass that crowd, he could not get through because they are like a chain. He would need to cut the connection between to of them, maybe using a power saw. Okay, sorry, forget this example – it was a little brutal. But the water particles cling just as close together an air can’t just simply slip through this. Take to needles and hold them close together with a tiny slit between them. Then place a water drop on top of the two needles and get them very slowly apart. It will stay there for a few millimetres and can’t pass the slit. This property of water is called a strong “surface tension”. The weaker this surface tension is, the easier can air or other substances pass through the surface. The surface tension of water is big enough and the slit between ball and bottle small enough that air can’t penetrate the surface and get into the bottleneck.
By the way:
Mr Bernulli was not a zero, although his name may sound like he was. He was the man who first discovered and described this principle: The faster a gas or a fluid streams, the lower is its pressure.
The vacuum generated by a fast streaming fluid is used in many technical devices. Mr Venturi invented one of them. He built an exceptional tube where fluid or gas needs to stream much faster when the tube gets more and more narrow. This narrow point is in the middle of the tube, and there is very low pressure. So he drilled a hole into the tube just at this point. Guess what happened? The low pressure inside the tube sucks in the air or other gas or fluid into the hole from the outside, where the pressure is normal (means much higher). That is a sucking device, and when you use water as the fluid which streams fast through the tube it is called “water jet pump”.
A carburettor in a car motor uses such a tube: Air streams through it gets faster and draws in petrol- The air-petrol mixture streams into the cylinder. When the spark plug is fired, its spark causes an explosion of the air-petrol mixture. These explosions are the source of force which is accelerating the car and makes it move.
When you dive into the water, the surface tension is weak enough to let your body penetrate the surface. If it wouldn’t, you would end up with lots of bruises.
But more lightweight things which would not swim on top of the water may stay on the surface. You could try this with a tiny needle. If the surface tension is strong enough, the needle will stay on the surface for quite a long time.
There are animals which use the surface tension to walk on water. The pond skater is such a lightweight animal:
You can weaken the surface tension of water with dishwashing soap. Repeat our experiment with lots of soap in the water. You will find it very difficult to seal the bottle with the ball.
Where I’ve got the idea from:
„The Magic of Science“ von Tony Griffith, seminar booklet
What’s coming up:
Next week in SFK #7, we will build an electric worm and demonstrate an anti-gravity miracle.