033 Anti-Gravity Biscuit Tin

We expect the biscuit tin to roll downslope. Why?

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We assume that the biscuit tin has its center of mass (C.M.) at the center of the tin. If so, when the tin roll downward, the C.M. goes lower. So the tin loses GPE to gain KE. Everything makes sense.

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The biscuit tin in the video however has a C.M. that is off centre (because of the mass of the magnets). If the C.M. is positioned on the uphill side, the tin rolls upward, but the C.M. actually goes lower. So again, the tin loses GPE to gain KE. There is nothing “anti-gravity” about the tin’s motion.

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It is even possible for the tin to rest on the slope. This occurs when the C.M. of the tin is vertically above the contact point. The allows the contact force F_C  to balances the weight of the tin, and yet does not exert any moment about the C.M. of the tin.

• 04 Forces