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The standing doll

The standing doll
Materials and mounting
instructions:
  an oblong plastic container with a round bottom (which can fake a “doll”), easy to open and close, some putty or other heavy object to be used as ballast. Open the container, fix the ballast to its bottom and close it again.
How to play:   if you incline the “doll” with the ballast in its bottom, it will not fall, but, as you release it, it will go back to the vertical position, after oscillating for a while. If you incline it while empty, it will roll over and lie on its longer side. The same would happen to the doll with the ballast, if you put on its top a heavy “head”!
Physics:  
  • force and interaction: the weight (gravity force, W) is directed downward and it is balanced by the upward force R of the support plane,
  • the weight is distributed all over the doll, however we can imagine it to be concentrated in a point b which is called the barycentre,
  • when we incline the “doll” with the ballast in its bottom, the gravity force tends to restore the vertical position since it is on the “stable side” with respect to the support point a,
  • the doll oscillates until it reaches the equilibrium position,
  • the empty doll (or the doll with a heavy “head”) would have a higher barycentre and, by inclining it, it would fall on the opposite side.
Educational aspects:  
  • discover force and interaction (Fa),
  • explore the importance of the weight distribution to determine the barycentric position (Fc),
  • explore the conditions for stable and unstable equilibrium (Fe),
  • observe oscillations and periodic motion (Se),
  • compare with your expectations.
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Last modified: Thu Nov 16 10:17:04 CET 2000