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Colloque Promenade dans la physique d’aujourd’hui / A Stroll through Today’s Physics

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Snail balls and more
William R. Young (Scripps Institution of Oceanography, U.S.A.)

29 juin 2007

I’ll consider the dynamics of a magic trick, the snail ball, which is marketed with the explanation: “A small metallic gold ball just over 2 cm in diameter ... the ball does roll, but does so incredibly slowly. To an audience it seems baffling... inside the ball, which is actually hollow, there is a viscous liquid and a smaller ball which is is the smaller heavier ball which determines the pace and this is slow because of the viscous liquid ”.
I’ll consider an experimental cylindrical analog consisting of a hollow cylindrical shell, a nested solid cylinder, and a gap filled with viscous fluid. This apparatus advances slowly and irregularly down an inclined plane. For small slopes the speed is constant on average.
A mathematical model is compared with simple experiments and the disagreement between theory and experiment indicates that the size of contact asperities plays a crucial role in determining the average rolling speed. This hypothesis is supported by coating the inner cylinder with sandpaper of different grades, which changes the rolling speed. Time and circumstances permitting, I’ll show other toys, involving granular materials, which roll slowly on inclined planes.

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William R. Young William R. Young (Scripps Institution of Oceanography, U.S.A.)