What you need:
- CD without scratches
- Balloon (not too big, preferable cigar-shaped)
- Piece of sponge rubber
- Big knitting needle or nail
- a push-pull cap of a water bottle
What you will get at the end:
Have you ever heard about a hovercraft? Have you already seen one? Once I have crossed the Channel from France to Great Britain with a hovercraft!
But what is a hovercraft? And what means to hover? Maybe it’s a vacuum cleaner? No – that’s a hoover. To hover means “to float in the air” and “craft” is another word for a ship.
But a ship does not float in mid-air, right? Well but a hovercraft does really float over the water surface. It rests on what is called a “hover cushion”. That’s a cushion made of air and nothing else. The ship rests on this air pillow over the water surface.
And what’s a hovercraft CD? That’s an ingenious toy: a CD sliding on a hover cushion. And this toy we’re going to build right now.
How to build:
All we need is such a water bottle’s push-pull cap:
And of course, we need a CD for a hovercraft CD. This CD is supposed to slide on its silver-shining surface later on. On the opposite side, where you can put a label on, we hot glue a small piece of sponge rubber. Make a trail of hot glue all around the centre hole of the cd. Then press the sponge rubber firmly on the CD. It is essential that no air can flow through any gap between CD and rubber. But watch it: don’t use too much hot glue because it must not squeeze out of the centre hole. This would ruin our toy!
Once the glue is cold, you have to punch a hole right in the centre through the sponge rubber. Don’t make it too wide because we couldn’t make it smaller later on. But if it’s too small, we could widen it a little bit later on.
Now you need to press the push-pull cap onto the sponge rubber with one hand and use the other hand to put hot glue all around the cap to glue it onto the rubber. Again we must pay attention that there is no air gap through which air might be able to escape.
Alright, now we going to start our hovercraft the first time. Push down the cap so that it is closed. Blow up the balloon and hold the nozzle closed while putting it on the cap. The air keeps inside the balloon because the cap is closed. Now position the CD on a very smooth surface (like a wooden floor) like in the picture (please note that in the picture I have used an old pen cap instead of a push-pull cap – that was the old method before I came up with the push-pull cap…):
Now pull the cap open and give the CD a little kick, and it will speed up like an ice skater.
Not with your hovercraft CD??? No problem: Most probably there is not enough air coming out of the hole to build up a proper hover cushion. Or maybe some dirt is underneath the CD? Sometimes little strands of hot glue tend to stick to the CD surface. You need to remove this dirt because our hover cushion is very thin – thinner than a millimetre. So the smallest piece of dirt will make the CD resting on the dirt instead of the hover cushion!
If all the dirt is removed and it still does not hover you may need to widen the centre hole of the sponge rubber. Take a nail or something similar to expand it just a little bit. Maybe your dad has to make the nail hot by using a flame to do this. But watch it! If the hole gets too wide the air coms out of the balloon very fast and your hovercraft CD will only hover for a second.
If the balloon is too big, it might force the CD to tilt a little bit when giving it a kick. This is also not good and you might try a different (smaller or cigar-shaped) balloon.
My hovercraft CD also did not work on the first trial. But then it worked great… Till the air was out of the balloon…
But why does this work?
How it works:
I explained the rocket with the backstroke engine last week. We’ve learned that there is a force that is slowing the rocket down. Do you remember its name? Alright, that’s the “friction force”. If you give a CD without the hover cushion a kick, it will only slide a few inches because the friction between the floor and the CD does slow it down. To keep the CD sliding, we would need to put force on it constantly. And because this is boring and exhausting, we have invented the hovercraft CD.
With the hovercraft CD, floor and CD are never touching one another. Air is flowing out of the balloon through the small centre hole and underneath the CD. The only way to get out of there is to stream all-around to the edge of the CD. The air is distributed equally underneath the CD before it emerges out from there. This streaming air does lift the CD a little bit. So the CD floats on its hover cushion. And such a hover cushion has a perfectly smooth surface. A smooth surface means very low friction. Low friction means just a very low force is slowing the CD down.
By the way:
The British Army has used such hover crafts in the desert where sand is a large friction force for wheels (remember me asking you to try a bicycle ride through sand?). Such huge hover crafts don’t use balloons to float over the ground. They have huge fans (called “blowers”, try to think of a giant hairdryer).
Hover cushions are also used in industry to move around big and heavy stuff. They are called “hover cushion platforms”.
By the way: friction does not only slow down moving things but also generates heat. When your family has made a long journey using your car, you can feel this when touching the tires. In ancient times people even made fire by friction! They were rubbing a wooden stick against another piece of wood until it got so hot that it could inflame chipped wood.
When learning what has slowed down our balloon rocket, we have learned something about another “braking force”: “Air resistance”. Part of this resistance is friction between the air and the moving object. When having a very high speed, this friction might produce lots of heat. This is the case when a spacecraft returns from outer space to the earth. When it dives into the air around the planet (which is called “atmosphere”) the aircraft is horrible fast and will be slowed down drastically. Parts of the spacecraft get so hot that metal would melt and needs to be protected by “thermal shields” made out of ceramic tiles.
Where did I find the idea:
What's coming up:
Next week, we will build an electrical motor from just three parts.