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Homework Help: Circular Motion help

  1. May 4, 2014 #1
    1. The problem statement, all variables and given/known data

    A chemistry lab centrifuge spins creating a circular trajectory for the solutes in the test tubes with a diameter of 20cm. If an acceleration of 10 times the Earth's gravitational acceleration is required, which of the following is a minimum frequency that must be achieved by the centrifuge? Hint: Circumference = 2∏r


    2. Relevant equations



    3. The attempt at a solution

    Ok, I tried using A=V^2/r then used V=2∏fr to solve for frequency but the answer I'm getting is not listed in the multiple choices. Am I missing a step or am I completely off track on how to solve this problem?
     
  2. jcsd
  3. May 4, 2014 #2

    berkeman

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    Staff: Mentor

    Welcome to the PF.

    Please post your math so we can check your work. Be sure to keep variables in mks units, and show what value you are using for "g" -- the acceleration of gravity.
     
  4. May 4, 2014 #3
    Thank you for making me show my work because I think I may have found my error. I'm using 9.8 as my value for g. The final answer I'm getting is 4.98 rev/s. Does this answer sound about right?
     
  5. May 4, 2014 #4
    I am getting a different result.
    Just type your work in. It is a rule.
     
  6. May 4, 2014 #5

    berkeman

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    Staff: Mentor

    I'd prefer to check your work rather than writing it out myself (I'm at work; been here all weekend). As a further hint, be sure to carry units along in your calculations, and cancel them out when you have the same units in the numerator and denominator. The LHS and RHS of your equation must have units that agree.
     
  7. May 4, 2014 #6
    No problem...

    a=v^2/r...

    9.8 x 10 = v^2/.1m = 9.8=v^2 = 3.13 m/s

    v=2∏fr...

    3.13=2(3.14)(.1)f = 3.13=.628f = 4.98 rev/s

    Options are...

    5 rev/s
    100 rev/s
    60 rev/s
    8 rev/s
     
  8. May 4, 2014 #7
    Seems right to me. I made a mistake.
     
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