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Q & A: Returning Coffee Can

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Most recent answer: 10/22/2007
Q:
I AM IN A SCIENCE CLASS FOR TEACHERS AND MY INSTRUCTOR DID A PHYSICS (?) EXPERIMENT AND WE HAVE TO FIGURE OUT HOW HE DID IT. HE HAD A FOLGERS COFFEE CAN WITH A HOLE AT EITHER END. A RUBBER BAND WAS COMING OUT OF EITHER END WITH A RUBBER BAND HOOKED TO IT. THE COFFEE CAN WOULD ROLE PART OF THE WAY DOWN THE COUNTER AND THEN COME BACK BY ITSELF. I THINK IT MIGHT HAVE SOMETHING TO DO WITH THE TIGHTNESS OF THE RUBBER BAND BUT I DON’T KNOW WHAT THAT DOES TO IT. COULD YOU HELP?
- RHONDA PUGSLEY (age 24)
UNIVERSITY OF NV, RENO, RENO, NV
A:
Dear Rhonda,
I cant say for certain, but I think I know how he did it.
Suppose you take a coffee can, make a hole at the center of each end, and put a rubber band through the can attaching it at each end. Now hang something from the center of the rubber band INSIDE the can, where you cant see it from the outside. If you hang something with an uneven shape, like a bolt suspended by it's head, it will keep the center of the rubber band from turning even if the outside of the can is rotated.

Now when you push the can across the desk, the outsides of the rubber band will turn with the can but the center wont, so the rubber band will be "wound up" as the can rolls. It will then try to unwind, which causes the can to roll back to you.

How does that sound?

(published on 10/22/2007)

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