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Vertical Circular Motion Question help?

  1. Jul 9, 2010 #1
    1. The problem statement, all variables and given/known data

    A 0.2kg mass attached to a rope 1.6m long is being spun in a vertical plane. Find the minimum frequency.

    2. Relevant equations

    F = 4(pi^2)(r)(f^2)(m)
    Ft min = Fc - mg

    - minimum frequency occurs at minimum tension


    3. The attempt at a solution

    I had this question on a test and spend like 45 minutes on it. I have no idea what to do, and I am curious as to what I should have done. I tried inserting numbers and re arranging formulas and substituting here and there and nothing seemed to work.

    HELP? D:
     
  2. jcsd
  3. Jul 9, 2010 #2

    ehild

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    What kind of force is F in your first equation?
    How much is the minimum tension?

    ehild
     
  4. Jul 9, 2010 #3
    F = 4(pi^2)(r)(f^2)(m)

    That F is centripital force. It can also be written as:

    F = m(v^2) / r
     
  5. Jul 9, 2010 #4
    Thinking physically, if the frequency is not high enough what happens? The mass will not make it to the top of the circle, right? It will execute free-fall motion (i.e., a parabola) until the string catches it again. So the key is to consider the top of the circle.

    What are the forces acting? Weight and tension. These combine together to provide the centripetal force maintaining the circular motion. The smallest the tension can be is zero. So we want to find the frequency in which the weight alone is able to provide the sufficient centripetal force.

    I think you can probably take it from here...
     
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