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The rotating button

The rottating button
Materials and mounting
instructions:
  a loop made of a thin string, about 1m long, threaded through two holes of a heavy button.
How to play:   hold the loop without straining it, with the button hanging in the centre; apply a rapid rotation to twist the string, then pull it, as shown in the figure: the button will start rotating alternatively in opposite directions. To maintain the motion, a slight kick, obtained with a pull-release movement of the hands, will be sufficient if given with the right synchronization (release when fully untwisted to allow twisting again, pull when fully twisted to allow untwisting).
Physics:  
  • different forms of energy:muscular, elastic (torsional under tension), kinetic;
  • energy transfer: from the hands to the string and to the button,
  • energy storage: as elastic energy in the string while the loop is being twisted,
  • force and interaction: the force is characteristic of the interaction between the hand and the string and we identify it by its “effects” (setting in motion),
  • action and reaction: while the hands apply a force to the string, the strimg applies in turn a force to the hands,
  • twisting and untwisting of the string is periodic in time, thus the tension variations must be synchronized in time.
Educational aspects:  
  • discover different forms of energy (Eb),
  • experiment transfers of energy (Ec),
  • experiment storage of energy (Ed),
  • identify the force needed to start and maintain the motion of the string (Fa),
  • analyse the relation between the force needed to start and maintain the motion and the energy transferred to the string (Ea),
  • understand the importance of a correct synchronization and estimate the time interval between pulls (Se),
  • compare with your expectations.
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Last modified: Wed Feb 14 11:46:28 CET 2001