How can I build a rubberband powered can?

Question

For physics, I have to make a can powered onlly by twisted rubberband roll 3.8 yds as fast as possible. The cylinder must be made of metal, only the cylinder or torque strusts may make contact w/ the round and the rolling of the cylinder must be means by which it is propeled there can be no devices on board that provide enery the racers cylinder must be in contact w/ the ground always and must roll due to friction w/ the ground and it must fit in a 12*12*12 inch box

Answer 1

When I first read your initial question, I thought that "can" was a typo for "car." That'll teach me to assume!

The simple solution is to spin a weighted arm inside a single cylinder, but the following is a little more challenging and I think the idea is really kind of neat -

Get yourself two cylinders that fit loosely together. The smaller cyinder should be heavier - glue some evenly spaced lead weights to the inside of it if necessary.

Put two small rods through the end of the smaller can so they cross like an "X". Cut the lengths as leave a short section of the rods protruding out of the sides of the cylinder. Attatch your rubber band to the center of this "X". Now place another lighter can on the outside (place the smaller, heaveir can with the "X" and the rubber band inside the slightly larger, lighter diameter can.)
Put another ""X" comprised of two more rods on the opposite end of this outside can. It is important that the can inside is much heavier than the ouside can so it will resist the rotational force of the rubber band and cause the lighter, outside can to spin against the floor. (The outside can will rotate against a small, flat ring placed against the rods protruding slightly from the inside can - a little grease might help this a lot)
Now simply connect the rubber band to the center of the "X" on the outside can and wind the cans up - set in on the floor watch 'er go!

Note: As for the physics' theory - the two cans will demonstrate equal but oppsite forces being applied by the rubber band, but because the mass of the center can is larger, the greater momentum will be present at the lighter, outside can in contact with the floor.

Answer 2

Attach an off-balanced weight to center of the rubber band.
Attach the two ends of the rubber band inside the can to the center of the two round bases of a cylindrical can. In this way nothing is protruding outside the can.
When you roll the can, the weight will make the rubber band coil just until the weight would start turning.
Put the can on a flat, horizontal surface and it will start rolling until the rubber band is uncoiled.

You must find the right weight experimentally.

Answer 3

You could use a variation of the old "spool tank" design. See the link.

I never made one of these that could travel 3.8 yards, but I'm sure it could be done.