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Moment of a rod of varying thickness

  1. Mar 23, 2012 #1
    1. The problem statement, all variables and given/known data

    Q A cylindrical steel shaft of radius 2cm and long is turned upside down on a lathe to one half its radius for a distance of 20cm from one end find the distance of its centre of gravity from the thicker end.?

    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Mar 23, 2012 #2
    Re: moment

    i dont get the question so can anyone help solve this
     
  4. Mar 23, 2012 #3
    Re: moment

    Hey,
    Welcome to PF.

    In the second line, is there a typing mistake?

    I think you have not copied the question in its exact language , which is causing some confusion (especially the grammer.)

    Can you retype it?
     
  5. Mar 23, 2012 #4

    tiny-tim

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    welcome to pf!

    hi lordsnowbrood! welcome to pf! :smile:
    (you missed out the length … let's call it L :wink:)

    do you mean that it has radius 2 cm for length L - 20 cm, and radius 1 cm for length 20 cm ?

    if so, find the mass of each part, pretend it's concentrated at the centre of mass (of that part), and then use the usual formula for two masses :smile:
     
  6. Mar 23, 2012 #5
    Re: welcome to pf!

    yes you are right.. but how to find mass??
     
    Last edited by a moderator: Mar 23, 2012
  7. Mar 23, 2012 #6
    Re: moment

    no mistakes buddy i copied it from my book Abbott physics
     
  8. Mar 23, 2012 #7

    tiny-tim

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    call the density "ρ" … it'll cancel out at the end :smile:
     
  9. Mar 23, 2012 #8

    gneill

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    Re: moment

    Wow! That must be Physics by Abbott and Costello then :smile:
     
  10. Mar 23, 2012 #9
    Re: moment

    I think you have not copied the question in its exact language , which is causing some confusion (especially the grammer.)
    Can you retype it?
    GRAMMAR
     
  11. Mar 24, 2012 #10
    i have made the changes can anyone now solve it
     
  12. Mar 24, 2012 #11
    Re: moment

    well it is a blue book published by heinemann and the author is abbott only
     
  13. Mar 24, 2012 #12

    tiny-tim

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    hi lordsnowbrood! :smile:
    yes, you can solve it! :wink:

    find the mass of each part, pretend it's concentrated at the centre of mass (of that part), and then use the usual formula for two masses …

    show us what you get :smile:
     
  14. Mar 24, 2012 #13
    how could i find the mass man ?? can u help me with that i can find volume but how to find mass in such question got no clue buddy
     
  15. Mar 24, 2012 #14

    tiny-tim

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    call the density "ρ" … it'll cancel out at the end :smile:
     
  16. Mar 25, 2012 #15
    could u please solve it coz i cant?
     
  17. Mar 26, 2012 #16

    tiny-tim

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    general strategy: write out the general formula first, then write it out again with the numbers in

    (that's why the homework template has that section on Equations! :wink:
    )​

    sooo …

    i] what is the general formula for the volume of a cylinder?

    ii] what are the volumes of these two cyliners?

    iii] what are the positions of their centres of mass (from the end of the large cyiinder)?

    iv] multiply by ρ to get the masses

    v] what is the general formula of the position of the centre of mass of two masses?

    vi] apply that formula (ρ will cancel) :smile:
     
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