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Homework Help: Circular Motion and friction problem.

  1. Jul 25, 2011 #1
    1. The problem statement, all variables and given/known data

    One end of a string is attached to a mass of 7.5kg which is at rest on a horizontal table, coeffecient of friction being = 1/3. String passes through a small hole in the table and supports at its other end a mass of 2.5kg which is revolving in a horizontal of radius 20cm. Find the number of revolutions made per minute if the mass on the table is on the point of slipping.

    3. The attempt at a solution

    I basically did Fc= 7.5*20* w^2
    and then
    Fc=T+1/3*7.5*9.8 and since T would be equal to 2.5*g i subbed that in to get


    From there equated the two Fc to get

    7.5*20* w^2 = 2.5*9.8+1/3*7.5*9.8
    And then i just found w and changed into revolutions per minute.

    Is this method correct?

    Oh yeah, hi guys as you can see i'm a first time poster and I assume I'll be using this forum, particularly this section, numerous more times in the near future.
  2. jcsd
  3. Jul 25, 2011 #2
    How did you got this?
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