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Torque required to rotate a disk

  1. Mar 3, 2012 #1
    1. The problem statement, all variables and given/known data

    How much torque is required to rotate a disk or diameter 1.4cm, height 1cm, weight 12g?

    2. Relevant equations

    I think

    Iz = mr^2/2

    Torque = force x distance


    3. The attempt at a solution

    Iz = 2.95 * 10^-7

    I do not know where to go from here.




    Thank you.
     
  2. jcsd
  3. Mar 3, 2012 #2

    tiny-tim

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    hi connor02! :smile:
    what is the rest of the question? :confused:

    (that's like asking how much force is required to push a disk of mass 12g)
     
  4. Mar 3, 2012 #3
    Hey Tiny-Tim,
    Sorry I didn't think any other part would be needed.

    You have a disc of diameter 1.4cm and height 1cm. The disc is connected to a rod of diameter 0.8cm and height 7cm. The disc and rod have a density of 7800kg/m^3. The disc and rod have been inserted into a barrel of diameter 1.4cm in order to extrude a fluid. You want this disc and rod to exert a force of 622.6 Newtons on the fluid. What torque should you rotate the disc and rod assembly by?

    There is a picture too, let me know if you need it and I will scan it for you.

    Thank you.
     
  5. Mar 3, 2012 #4

    tiny-tim

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    hey connor02!

    i'm still confused …

    is the barrel rifled (with a screw thread)?

    (the force is given in newtons, and the cylinder can be pushed, so where does torque come into it?)

    would you scan the picture? :smile:
     
  6. Mar 3, 2012 #5
    Hi Tiny-Tim,
    Yes the rod has screw threads at the top so the rod needs to be rotated for the rod and disc assembly to advance down the barrel. My scanner refuses to work so I have drawn the picture instead. The picture is attached here. Thanks.
     

    Attached Files:

  7. Mar 3, 2012 #6

    tiny-tim

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    ahh!

    then we need to know the pitch of the screw thread

    ie, how many turns per metre? :wink:
     
  8. Mar 3, 2012 #7
    Hi Tiny-Tim,
    That information is not given, does it need to be known? If it must be known, can you assume the pitch is x turns per meter and give the answer in terms of x?
     
  9. Mar 3, 2012 #8

    tiny-tim

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    well, you're asked to turn torque into force (newtons), so yes
    yes
     
  10. Mar 3, 2012 #9
    great, so how do I go about calculating it? I have Iz of the disc, now what do I do?
     
  11. Mar 3, 2012 #10

    tiny-tim

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    what do you think?
     
  12. Mar 3, 2012 #11
    I don't know how to apply torque = force x distance on a disc? And where the Iz value comes in? I only calculated Iz cus the hint of the question gave the Iz formula for the disc.
     
  13. Mar 3, 2012 #12

    tiny-tim

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    i don't think the mass or the moment of inertia of the disc is relevant …

    it isn't being accelerated, so it doesn't itself need any force or torque

    (and i'm going out now)
     
  14. Mar 3, 2012 #13
    Okay.

    Could someone help me out?

    Thanks.
     
  15. Mar 5, 2012 #14
    help?
     
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