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Q & A: Rolling cans

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Most recent answer: 10/22/2007
Q:
Hi, im doing a physics assignment looking at rolling objects down a surface and then comparing. I am really confused because i was under the impression that of the two cans i rolled down a 1 metre slope (one hollow and one full) that the full can would roll faster. In my results they seem to be equal or occasionally the hollow can rolls fast. Am incorrect in saying that a heavier object with the same diamiter should roll faster?. yours sincerely monte.
- monte
sydney, Australia
A:
According to the standard physics lore, the full one should roll faster if the contents are solid.  That's because the moment of inertia relative to its weight is less than that of the empty can.  (That means more of the gravitational energy is converted to downward motion and less to turning motion.) All bets are off if the cans don't really 'roll without slipping' but have also a skidding component.
An additional complication is that if the can contains a not-very-viscous liquid, like thin soup, then the liquid doesn't contribute to the effective moment of inertia but does contribute to the mass and overall downward force.   Try rolling a can full of rocks or other solid material like spam.

LeeH (w. Mike W)

(published on 10/22/2007)

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