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B Torque and forces parallel vs perpendicular to the axis of rotation...

  1. Jan 16, 2017 #1
    why force parallel to the axis of rotation do not make any torque.
     
  2. jcsd
  3. Jan 16, 2017 #2

    Nidum

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    What is the usual definition of a torque ?
     
  4. Jan 16, 2017 #3
    is the tendency of force to rotate object around an axis .
     
  5. Jan 16, 2017 #4
    or it's the cross product between the force and the distance vector .
     
  6. Jan 16, 2017 #5

    Nidum

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    Can you answer your own question now ? How can a force acting parallel to an axis tend to cause rotation about that axis ?
     
  7. Jan 16, 2017 #6
    it can't
     
  8. Jan 16, 2017 #7
    but way .
     
  9. Jan 16, 2017 #8

    Nidum

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    Torque demo.jpg

    In the diagram :

    A-A is the axis
    B-C is a lever arm at right angles to the axis
    Fa , Ft and Fr are axial , tangential and radial force components acting on the end of the lever arm at point C .

    Which of the force components could cause rotation of the lever arm about axis A-A and which could not ?
     
    Last edited: Jan 16, 2017
  10. Jan 16, 2017 #9
    i think Fr
     
  11. Jan 16, 2017 #10
    can any one help me plz
     
  12. Jan 16, 2017 #11

    cnh1995

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    No. Look at the diagram carefully.
     
  13. Jan 17, 2017 #12
    Fr don't make torque becuase it is perpendicular to axis of rotation.
    Fa i think
     
  14. Jan 17, 2017 #13
    can any one help me
     
  15. Jan 17, 2017 #14
    Ft wiil cause rotation about AA axis.....
     
  16. Jan 17, 2017 #15

    Nidum

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    Correct . Can you explain in words now why Ft does produce rotation about A-A axis and why Fa and Fr do not ?
     
  17. Jan 17, 2017 #16
    Look t=r×f......the anlge btween r and fr is sin0=0...so torque is zero....no question for rotation.....
    However fa will produce rotaion but the axis will be different....(an axis which is perpendicular to AA......BUT NOT ALONG AA)
     
  18. Jan 18, 2017 #17
    So fa will rotate the object
     
  19. Jan 18, 2017 #18
    Yes obviously fa will.....but as i said corresponding rotation axis will be different
     
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