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The balance

The balance
Materials and mounting
  a support basis (about 10x20 cm2) a rigid balance arm (about 5x40 cm2), a rigid cardboard cylinder, two rigid plastic saucers, two small washers, two nails, a few arbitrary “weighing units”. Fix the saucers at the opposite ends of the balance arm, in symmetric positions (to improve the definition of the support point glue the two washers to the underside of the saucers). Fix the cylinder to the support and, on its top, fix the balance arm, centring it in the best way: use the two nails to maintain the central position, but be sure that they leave the balance arm free to oscillate. One can also make an unequal arm balance by fixing one saucer half way between the centre and the end.
How to play:   check that the balance is in equilibrium with empty saucers (if it is not, improve it by adding a small weight on the lighter side). To weigh properly, put the object on one side and the “weighing units” on the other, adding them gently one at time until equilibrium is reached.
  • this is an “equal arm lever”: the two arms being equal, the equilibrium is reached when the two weights on the two sides are equal: the balance thus makes a direct comparison of weights,
  • the balance makes also a direct comparison of masses, since equal weights (that is equal gravity forces) correspond to equal masses,
  • to measure the mass, one needs “weighing units”, which may be arbitrary but must be all equal, in order to express the weight as the number of weighting units needed to balance,
  • before reaching the equilibrium, the balance oscillates around the equilibrium position with a periodic motion.
Educational aspects:  
  • understand that masses can be compared using an instrument (Sd),
  • discover force and interaction and different types of forces (Fa),
  • discover equilibrium of forces (Fe),
  • analyse the equilibrium conditions with an unequal arm balance (Fe),
  • observe oscillations in a periodic motion (Se),
  • compare with your expectations.
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Last modified: Thu Nov 16 10:49:55 CET 2000