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Homework Help: Rotational Motion

  1. Aug 1, 2008 #1
    1. The problem statement, all variables and given/known data
    A rock of mass m=2kg is tied to a string of length r=4m is rotating at a constant rate and subject to a centripetal force Fc=32N.

    Find the:
    a)Centripetal acceleration

    b)Angular velocity

    c)Angular momentum


    2. Relevant equations

    a) Ac=V2T/R (Fc=V2T/R; so, V2T=Fc x R/m?)

    b) VT=angular velocity x radius

    c) l = mvTr

    3. The attempt at a solution

    a) 32 x 4/2 = 64[m2/s]

    b) V2T = 64, so VT = 8; 8/4 = 2 [rad/s]

    c) 2 x 8 x 4= 64[kg x m2/s]

    I know this is simple stuff, but I'm new to physics, and just wanted to double-check that I'm getting my formulas right. Thanks everyone!
     
  2. jcsd
  3. Aug 1, 2008 #2

    rock.freak667

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    Homework Helper

    Use Newton's 2nd law here.
    [tex]F_{centripetal}=ma_{centripetal}[/tex]

    This is correct. You could have also used [itex]F=m \omega^2r[/itex] if you wanted.

    [/QUOTE]

    This looks good as well.

    I know this is simple stuff, but I'm new to physics, and just wanted to double-check that I'm getting my formulas right. Thanks everyone![/QUOTE]
     
  4. Aug 1, 2008 #3
    Wait, it looks like I may have jumbled up my formulas...

    If V2T=Fc x R/m (is this formula correct?), if so, then V2T = 64;

    Ac=V2T/R so 64/4 = 16 [m/s2]

    Am I still missing the mark?
     
    Last edited: Aug 1, 2008
  5. Aug 1, 2008 #4

    rock.freak667

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    Homework Helper

    16ms-2 should be correct
     
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