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What is the minimum frequency to keep the mass moving?

  1. Oct 31, 2014 #1
    1. The problem statement, all variables and given/known data
    A mass of 2.0 x 10^2 g is tied to a 1.6 m long string and spun in a vertical circle. What is the minimum frequency to keep the mass moving?

    2. Relevant equations
    Fc=m4(pie)rf^2

    3. The attempt at a solution
    I know I have to use this equation and make it equal to another to find Fc however I don't know which other equation.
     
  2. jcsd
  3. Oct 31, 2014 #2

    BvU

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    Well, perhaps this isn't done in outer space but on earth...
     
  4. Oct 31, 2014 #3

    haruspex

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    That equation is not quite right. (And it's spelt pi, not pie.)
     
  5. Oct 31, 2014 #4
    Is the equation Fc = m4(pi)^2rf^2
     
  6. Oct 31, 2014 #5

    haruspex

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    Yes.
    Now what about BvU's point? You need to consider gravity.
     
  7. Oct 31, 2014 #6
    Would Fg = Fc?
    So I could get mg = m4(pi)^2rf^2. Then solve for f?
     
  8. Oct 31, 2014 #7

    haruspex

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    Yes, but is that just a lucky guess or can you mount an argument to say it should be that?
     
  9. Oct 31, 2014 #8
    I drew a diagram and I think that when the mass is at the top it will need the minimum Fc as gravity will help bring it down. Where at the bottom the Fc will have to overcome gravity to bring the mass back up. So gravity will be the only force acting on the mass and bringing it down when it is at the top.
     
  10. Oct 31, 2014 #9

    haruspex

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    Good.
     
  11. Nov 1, 2014 #10
    Thank you so much! Now it seems simple since I understand it
     
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