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The gravity point
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Materials

      Potato.
      2 metal forks.
      Toothpick.
      Glass.
 

 
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Procedure

     1. Select a potato that is more or less symmetric.

     2. Spit the toothpick in the potato taking in account the symmetry axes of it. (the placement of the toothpick should be in the vertical symmetry axis and a little bite above the horizontal symmetry axis)

     3. Spit the forks in the potato, in a way that the toothpick is between the forks. (the vertical symmetry axis of the group should be maintained)

     4. Place the toothpick in the glass border and adjust the angle between the forks until the system is in equilibrium. (if the group falls, reduce the angle between the forks. If, in contrast, the group falls inside the glass, increase the angle between them)
 
Why?

The toothpick, potato and forks system is in equilibrium in the border of the glass! As well as the circus equilibrists, our system is suspended in the border of the glass without falling. This fact is due to the existence of a virtual point in the toothpick, where the whole mass of the group is "concentrated". When we support that point in the glass border, it is as if we have placed there a virtual object with an equivalent mass.
In the system of the present experiment, the mass gravity center is located in the toothpick because the potato weight is compensated by the forks weight. Thus, when you want to equilibrate yourself, find the intersection of your symmetry axes and move you until you have that point above the place where you want to stay. Try it, but don't hurt yourself!
 




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