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Solve rpm in rotational motion problem

  1. Mar 14, 2008 #1
    1. The problem statement, all variables and given/known data
    How fast (in rpm) must a centrifuge rotate if a particle 7.0 cm from the axis of rotation is to experience an acceleration of 100,000 g's?

    My variables
    radius= 7 cm or .07m
    radial accel= 100,000 g's or 980,000m/s^2 or 98,000,000cm/s^2
    V= I think I need this to get w
    w= ? I think I need to solve this to get rpm
    rpm= ? this is what I want

    2. Relevant equations
    radial accel = v^2/r
    v= rw

    3. The attempt at a solution
    980000/.07 = 68,600

    [tex]\sqrt{}68600[/tex]/.07= 261.9
    261.9/.07 = 3,741 rpm

    whether i set everything as meter or centimeter, I still get the same answer. I check the back of the book and it says 3.6x10^4 or 36,000rpm
    1. The problem statement, all variables and given/known data

    Any help would be appreciated.
  2. jcsd
  3. Mar 14, 2008 #2


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    Science Advisor
    Homework Helper

    Your answer is correct but it's in radians per second. rpm is revolutions per minute. A revolution is 2*pi radians and a minute is 60 seconds. Can you convert it to rpm?
  4. Mar 15, 2008 #3
    thank you, I did:
    224,460/2*pi=35,723rpm or 36000rpm if I round up. Thanks again!
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