Winding Rubber Band How could you store the kinetic energy of a soda-bottle using a rubber band so that it unwinds after being rolled across a flat surface?
Basically, how can you make a bottle roll back after being pushed across a flat surface.
(preferably using a rubber band)
 A: Glue one end of the rubber band to the bottom of the bottle (on the inside) and the other to the inside of the cap.  (It may be easiest to cut the bottle open for this and later tape it back together, but with sufficient dexterity and suitable tools, you could do this through the neck of the bottle.)
Attach a weight to the middle of the rubber band, in such a way that the weight can't rotate around the rubber band without twisting it.  Make sure the band is short enough that the weight hangs suspended when the bottle is horizontal.
When the bottle is horizontal, gravity will keep the weight below the rubber band, and will thus prevent the middle of the band from turning.  Thus, when you roll the bottle on the floor, the band will twist, storing energy.  When you let go, the rubber band unwinds, making the bottle rotate backwards and thus roll back to where it started.
Of course, there's a limit to how much energy you can store this way.  If you roll the bottle far enough, the tension in the rubber band will eventually overcome gravity and set the weight spinning (or the band will snap).
