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Fifi's Craft - Cotton Reel Tractor - 20 Nov

Cotton Reel Tractor

Here's an oldie but a goodie- takes me back as far as my fathers schooldays!
A nice little science project too.

You will need:

- Medium sized or larger cotton reel. Wooden ones are the best but if you can't find one a plastic one will do.
- A strong rubber band
- Pencil
- Half a matchstick
- A standard candle (10mm round)
- Glue or tape
- Optional: paint to decorate it with

1) Cut a few 7mm slices off the candle, pulling out the wick from the centre as you do. These are going to be wax washers. Make a hole through the middle of each washer- the easiest way to do this is to heat a skewer and melt your way through. You will only need one washer but it's handy to have a few spare.
2) Thread the rubber band through the middle of the cotton reel and slip the half matchstick under it before you pull the band all the way through. Glue or tape the matchstick to the reel so that it doesn't move.
3) Pull the rubber band out the other side- a small piece of wire or a paperclip with the end bent into a hook makes this much easier to do.
4) Thread the rubber band through the wax washer and pull it out so that the reel and washer are sitting next to each other, held together by the band. Slip a pencil through the rubber band loop, about 3cm from the sharp end.
5) The loop should be tight but not strained. If the pencil flops about it is too loose. If you rubber band is too long, tie a knot in the end.
6) Now hold the reel in your hand and wind the pencil round and round to twist up the rubber band inside, then set it down onto a flat surface and watch the tractor move. It will crawl along the surface quite slowly.

If you want to decorate your machine, paint the reel and pencil first.
Make a few of them and have races. A simple and effective toy.

Science fact: As you turn the matchstick, you twist and tighten the rubber band. The unwinding band releases the energy stored in the twisted rubber and makes the tractor move. Try different washers- does cork make a difference to the speed? How about plastic or metal? What does this tell you about friction?