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Homework Help: Please help I'm stuck

  1. Nov 7, 2004 #1
    problem with static friction and force

    I have been working on this problem all weekend and I just can't get it!!

    A small object is placed 10cm from the center of a phonograph turntable. It is observed to remain on the table when it rotates at 33 1/3 rev/min but slides off when it rotates at 45 rev/min. Between what limits must the coefficient of static friction between the object and the surface of the turntable lie? Calculate the value of Fc.

    I believe I found the first part of the question by taking Us=v^2/g*r for each value that it rotates. I think that I need to use the formula Fc=m*v^2/r for the second part of the problem but I have no clue how to find the mass and have looked all through my notes and physics book. Please if you could help me I would greatly appreciate it.
    Last edited: Nov 7, 2004
  2. jcsd
  3. Nov 7, 2004 #2
    What is Fc ?
  4. Nov 7, 2004 #3
    I believe it is the force
  5. Nov 7, 2004 #4
    its centrip force
    Last edited: Nov 7, 2004
  6. Nov 7, 2004 #5
    it is actually F sub c in the problem....centripetal force
  7. Nov 7, 2004 #6
    yeah it is centrip force can anyone please help!!
  8. Nov 7, 2004 #7
    are you allowed to write Fc in term of m?
  9. Nov 7, 2004 #8
    yeah I am pretty sure I am
  10. Nov 7, 2004 #9
    then just go ahead with it. give the range of Fc using the range of your coefficient of static frictional.
  11. Nov 7, 2004 #10
    how do I go ahead with it?? I don't know the mass
  12. Nov 9, 2004 #11
    i thought you said that you were allowed to write Fc in term of m ?
  13. Nov 10, 2004 #12

    Just start working on the problem using a symbolical "m" for mass. If the figure is not in the original problem, this usually means it will cancel out somewhere.

    Think of it as a nice aid to proove your solution: if the mass doesn't cancel out in your solution, you might be doing something wrong...

    I usually only fill out the numbers once I have a general solution written out in symbols.

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