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Homework Help: Calculating Centrepetal Force

  1. Aug 27, 2011 #1
    1. The problem statement, all variables and given/known data

    Calculate the centrepetal force required to rotate a3kg object in a circle at a radius of 3m at one revolution per second.

    2. Relevant equations

    MV2/r
    2*pie*r

    3. The attempt at a solution

    First i presume i must convert the revolution per second into metres per second.

    Calculate the circumference.

    2*pie*r
    2*pie*3
    18.84955592153876m

    Since its one revolution per second then it would mean:

    18.84955592153876ms-1

    Now for centrepetal force:

    mv2/r

    3*18.84955592153876 squared/3

    1065.9172753176507952118585470128 / 3

    Centrepetal force = 355.31N (2dp)


    I hope this is right, the circular motion stuff still has never sunk in

    Thanks, Joe
     
  2. jcsd
  3. Aug 27, 2011 #2

    rock.freak667

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    Homework Helper

    Your calculation of the tangential velocity v is incorrect, you should use v= rω and then put that into F=mv2/r or F=mω2r for a more direct approach.
     
  4. Aug 27, 2011 #3

    Doc Al

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    Staff: Mentor

    Looks good to me. :smile:
     
  5. Aug 27, 2011 #4

    Doc Al

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    Staff: Mentor

    Why do you say that?
     
  6. Aug 27, 2011 #5

    rock.freak667

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    Nevermind, my bad, I used rpm instead of what it was rps.
     
  7. Aug 27, 2011 #6
    That's quite right. I would have written 355.3 N, but that's just my "three significant figures" training.

    MIT Open Courseware has an excellent lecture series on Classical Mechanics, with a great video on circular motion.
     
  8. Aug 27, 2011 #7

    PeterO

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    Homework Helper

    There are many texts and references that show it, but if you look at the following wiki reference - the formulas right at the start - you will see that there is another formula for centripetal force that can be used in exactly this situation - when you know the Period of rotation rather than how fast it is travelling. That means you can't make a mistake calculating v, because you never calculate it!

    http://en.wikipedia.org/wiki/Centripetal_force
     
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