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Charged Beads on a Spring

  1. Mar 19, 2008 #1
    The question states:

    "You have a lightweight spring whose unstretched length is 3.28 cm.
    You're curious to see if you can use this spring to measure charge.
    First, you attach one end of the spring to the ceiling and hang a 2.57 g
    mass from it. This stretches the spring to a length of 4.37 cm. You then
    attach two small plastic beads to the opposite ends of the spring, lay
    the spring on a frictionless table, and give each plastic bead the same
    charge. This stretches the spring to a length of 3.78 cm. What is the
    magnitude of the charge on each bead?"

    So, after finding the spring constant using the mass hung from the
    ceiling, we can calculate the restoring force of the spring in the
    spring-2bead system. This restoring force must be equal to the electric
    force exerted by the beads since the spring is at equilibrium. This is
    the part that I'm having trouble with. Shouldn't the restoring force be
    equal to 2x the electric force? Each bead will exert a force on the
    other and cause each to stretch the spring in opposite directions. In other words, the electric force of each bead on the other (so there are two forces) is what actually causes the spring to stretch to its equilibrium length. If we calculate using only one electric force, aren't we making the assumption that one bead is held still in position?

    The answer as it stands is found by equating the restoring force to only one electric force so where have I gone wrong?

  2. jcsd
  3. Mar 19, 2008 #2


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    … springy thingy …

    Hi nothing123! :smile:

    But if one bead was held still in position, wouldn't the length of the spring be the same? :smile:
  4. Mar 19, 2008 #3
    Yes, you're right and I have reasoned it this way. Nevertheless, if one wasn't held in place, why would the result be the same?
  5. Mar 19, 2008 #4


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    Why shouldn't it be? :confused:

    How could any other part of the spring "know" what was happening at one end? :smile:
  6. Mar 20, 2008 #5
    We are applying Newton's second law on the Bead. There are only two forces working on it-electrostatic and spring force. So [tex]F_s=F_q[/tex] not [tex] F_s=2F_q[/tex]

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