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Homework Help: Force on a massive ring

  1. Jul 21, 2011 #1
    1. The problem statement, all variables and given/known data
    A massive ring (radius = a and mass = M) lies on the xy plane. Calculate the force F on a test mass m at position z on the z axis.
    Now assume a = 1 and GMm/a2 = 1 in some system of units.

    (a) What is the force on m at z = 0?
    I got the answer for this its 0
    (b) What is the force on m at z = a?
    the answer for this is 0.354
    (c) What is the maximum force on m ?
    I am stuck on this one.

    2. Relevant equations


    3. The attempt at a solution

    after some work I get F=(2GmMx)/(x^2+a^2)^3/2
    i know this is right because i got part b from it, however i dont Know how to find the maximum, my gut feeling is take the derivative with respect to x and set it equal to zero and then solve for x and plug back into F
  2. jcsd
  3. Jul 21, 2011 #2


    User Avatar

    Staff: Mentor

    I'm not sure where that "2" comes from in your force expression. Better check that derivation.

    And yes, you need to differentiate and solve as you've outlined.
    Last edited: Jul 21, 2011
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