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Playing a bathroom scale

Playing a bathroom scale
Materials and mounting
instructions:
  two bathroom scales, a calibrated rubber band, a folding ruler.
How to play:   first measure with the scales the weights of two kids, then let one of the kids pull down a robust rubber band, while the other holds it: the weight of the kid who pulls down will diminish, the while the weight of the other will increase by the same amount. With the folding ruler, measure the length of the rubber band and check that the applied force is equal to the variation of the gravity force of each kid.
Physics:  
  • force and interaction: when the kids simply stand on the bathroom scales, the scales measure their weights, that is the gravity forces Wb and Wg,
  • the gravity force has the direction of the vertical line,
  • when the boy pulls down the rubber band, he applies to the band a downward force, but the rubber band applies to him an upward force Rb (action and reaction),
  • the composition of the two opposite forces gives a resultant force smaller than Wb,
  • when the girl pulls up the rubber band, she applies to the band an upward force, but the rubber band applies to her a downward force Rg (action and reaction),
  • the composition of the two forces gives a resultant force larger than Wg.
Educational aspects:  
  • discover force and interaction and different types of forces (Fa),
  • discover direction of forces (Fc),
  • discover action and reaction (Fd),
  • analyse force composition (Fc),
  • measure the force with a calibrated instrument (Fb),
  • compare with your expectations.
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Last modified: Sun Feb 25 13:07:08 CET 2001