Hands-on Science Carnival 2010 Activity Stations: General Physics

Forces (Air Pressure, Weight and Friction): CD-ROM Balloon Hovercrafts

[Shopping list: CD-ROM disks; reinforced packing tape; thin plastic film (from packaging); white PVC pipe ~3/4" DIA; clear flexible PVC tubing that will fit tightly inside the rigid white pipe; rubber bands; 9" latex balloons; push-pins; small squares of ~120-grit sandpaper; hot glue guns and glue sticks; power strips and extension cords]

1. Pre-cut the white PVC pipe into 1-1 1/2" lengths; the clear PVC tubing into 2" lengths, and the plastic film into 1x1" squares (if used).
2. Refill and warm up the glue guns.
3. Place a CD-ROM disk on the table with the printed side down (the side with the colorful grooves should be up). Run a continuous bead of hot glue around the hole in the center of the disk, then add more to cover the area of the squares of plastic film. Quickly press a plastic film square onto the glue and hold firmly a few seconds until the glue sets. [Alternatively, you could cut and tape a small square of reinforced packing tape over the hole in the disk (the kind with the parallel strings between the plastic layer and the adhesive. Don't use plain clear packing tape (it rips too easily) or Gorilla Tape (the stuff with two perpendicular layers of string reinforcement; the glue won't stick well to the plastic they use on this brand).]
4. Use a piece of sandpaper to round off the inside edge of one side of the white PVC pipe collar. This is to make it easier to insert the clear PVC tubing, so test fit it until you are satisfied that you can easily push the tubing in about 1/4".
5. Apply a bead of hot glue to the opposite end of the white PVC collar (i.e. the side that you did not sand), and quickly center it over the hole in the CD-ROM disk and press it firmly onto the plastic film square (or tape). Hold a few seconds until the glue sets, then let cool for about a minute. Carefully test it to be sure you have a strong joint- if it breaks, peel off the glue and repeat until it's strong.
6. For added strength, run a hot glue bead around the entire perimeter of the PVC collar where it joins the plastic square (or tape). Set aside to cool.
7. Once cooled, turn the CD_ROM disk over (balancing it on the PVC collar), and carefully poke 1-3 holes in the plastic film square (or tape).
8. Place a 9" latex balloon over one end of the clear PVC tubing, then wrap a rubber band several times to help hold the balloon in place.
9. Place the CD-ROM disk assembly (without the balloon installed) onto a smooth tabletop or clean linoleum or wood floor. Give the disk a push. Does it go very far?
10. Inflate the balloon by blowing through the clear tube, pinch the balloon neck above the tube to hold the air, and carefully insert the end of the tube into the white PVC collar. Push the tubing straight in, trying not to apply too much bending torque to the PVC collar, as the collar-disk joint will always be somewhat fragile.
11. Now give the hovercraft a push and see how far it goes.

What's Happening: Anything that slides or rolls across a surface eventually stops (or may be difficult or even impossible to start moving) due to friction between the surfaces in contact. A hovercraft moves easily by reducing the frictional forces between itself and whatever is underneath, in this case the table or floor. The amount of friction depends on the total surface area, the weight of the craft, and how smooth the materials at each surface are. In this case both the bottom of the disk and the table top are pretty smooth, but the weight of the hovercraft is still large enough to produce a lot of friction, so the craft cannot slide very easily or very far. We could reduce the friction some by adding a slippery layer between the two surfaces, such as wax or oil, or even water or ice, but that would be messy. Instead we can use air pressure from the balloon to push or lift the disk surface off of the tabletop. The amount of lift depends on the air pressure, which in our case depends on the pressure in the balloon and the holes in the plastic film which let it escape. It's not enough to fly, but it is enough to significantly reduce the surface area of the disk that is actually touching the tabletop at any given moment (also lowering the net force pushing the disk down), which reduces the friction force to nearly zero and allowing the hovercraft to almost float. Even a tiny push sideways can send the craft drifting for several feet with so little friction to stop it. It's like an air-hockey puck. Of course once the air in the balloon runs out, the friction returns and immediately stops the hovercraft.

Variations and Experiments: Try poking more or larger holes in the plastic film (or tape). If you poke too many, you can carefully cover them with new tape (make sure it stays inside the hole in the disk), and re-poke only a few new ones. How does this affect the speed of the hovercraft. How does it affect the total time it can operate? What about the size of the balloon?