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Why wouldn't the disc/paper rotate?

  1. Mar 31, 2015 #1
    I was playing around a piece of paper by placing my pencil in the center and applying torque on the pencil(pencil Y-axis paper x-axis vice versa), and that torque transmits to the paper as well and it starts to rotate. Then I moved the pencil to the far edge of the paper and applied torque there as well, the page rotated around the pencil. Now I placed another pencil to the opposite position of the first pencil (imagine the first pencil being in the far right of the paper and the second in the far left), then applied torque as well the page will not rotate. I started to think, if I did apply equal torque in the same direction to these pencils based on their positions the paper will not rotate, even on other shapes like a disc it will not. Why is that..? Why can't two different positioned points applying the same torque rotate the paper/disc? It seem as if they cancel out instead of adding up although they are in the same direction.
  2. jcsd
  3. Mar 31, 2015 #2


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    Are you sure you have accounted for all the forces involved in each trial? I can easily imagine differing frictions.
  4. Apr 1, 2015 #3
    Well we could ignore my experiment(because it completely in-accurate). And focus more of the theory, because me playing around generated that question.
  5. Apr 1, 2015 #4
    Can you clarify what you mean when you say you applied torque to the pencil? Are you spinning the pencil around its longitudinal axis? (If so, I doubt if there is enough friction at the point to rotate the paper, unless you are placing the eraser end on the paper)
    There is a good chance I have misunderstood you, but if that is what you are doing, then placing another pencil on the paper and doing the same thing, the paper will not rotate because a 2 Dimensional rotation (of a single object) can only take place in a single plane and only around a single point at any one time.
    If you are doing something differently from what I wrote above, please explain what it is.
  6. Apr 1, 2015 #5


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    In the case of the two pencils, any torque generated by twisting of the pencils is opposed by the torque from the person holding the pencils in place, since the pair of pencils held in place prevents the paper from rotating.
  7. Apr 1, 2015 #6


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    Let's break those forces apart: apply a torque with one pencil and hold the other pencil in place. Now do you see the linear force the second pencil applies?
  8. Apr 1, 2015 #7
    I think the best thing to do is re-state my question using diagrams, and most importantly making it accurate!
    So here it goes, still using the same objects a disc and a paper, and instead of my hand I'll place two small motors(attaching it mechanically to the objects) so that it could transfer controlled & calculated torque from the rotor/gear to the object, and the center of the object on a axis of rotation, as diagrammed here:



    m(1)/m(2) being the motors.

    And I placed the objects within a rotation axis, then powered the motors. The objects should/shouldn't rotate?
    My guess is they would not, since the torques are not applied via the rotation axis, also the possibility of the motor's torque being internal force?
  9. Apr 1, 2015 #8


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    You are correct that the system does not rotate, and we did understand it before....so, have you put any thought into the responses you got? Can you draw the rest of the forces into your diagrams?
  10. Apr 1, 2015 #9
    Not really, I'm having trouble when drawing FBDs and the force vectors... I don't know if I'm right or wrong :cry:
  11. Apr 1, 2015 #10
    I was using the eraser end.

    You understood me well, im still processing what you mean by the 2D rotation taking place in a single plane and only around a single point at any time do you mean that the two torques must be applied to one single rotation of axis?

    A bit confusing, what do you mean by the torque generated by the person holding the pencils in place?
  12. Apr 1, 2015 #11


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    Well, this is why I suggested focusing on one applied force at a time. That's what you need to do when drawing a FBD anyway.

    The point of a FBD is that all forces sum to zero. So if there is a torque applied by a motor in one direction (and the whole object isn't rotating), then a torque must be applied in the other direction, somewhere else. So....where is that other force applied and what is its value?
  13. Apr 2, 2015 #12
    I agree.

    Well, the motors apply torque on the objects as an action, and an equal an opposite reaction would be an opposite torque on the motor's stator?
  14. Apr 2, 2015 #13
    You ARE applying the opposing torque by holding the pencils firmly in place and therefore generating the normal force(see picture) ImageUploadedByPhysics Forums1427960149.520350.jpg
  15. Apr 2, 2015 #14

    What if it we're mechanically attached to it(in any efficient way), the source that is fixing it to place will generate a normal force correct(i.e screw,etc...)?

    The act of "fixing" it to the disc will generate a normal force?
  16. Apr 2, 2015 #15
    Say only one of the pencils attached was rotating and applying torque on the disc. The other pencil, which just stands idle, would then present a physical obstruction to the disc's movement and not let the disc move. It will do it through the use of normal force. This normal force will only exist while the torque is being applied and will only "pressure" the idle pencil (which will be balanced by normal force and balancing torque provided by pencil's base so the pencil doesn't move) until the torque is being applied.
    If both the pencils rotate they provide each other's balancing torque through their respective normal forces. Too much torque, assuming there's no slipping, will break the pencils.
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