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M=Fl solve a Moments problem (force X distance)

  1. May 25, 2016 #1
    1. The problem statement, all variables and given

    http://imgur.com/kxALo8w


    with how much force is the wire pinched?

    2. Relevant equations
    M=Fl

    3. The attempt at a solution

    I don't know how I am supposed to do this. This is my attempt:

    0,15 * 20 = 3Nm

    and now what?
     
    Last edited by a moderator: May 25, 2016
  2. jcsd
  3. May 25, 2016 #2

    mfb

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    Use the same formula again. This time you know torque and distance and want to get the force.
     
  4. May 25, 2016 #3
    but the force is 20 and 20 so I donät know what to do..
     
  5. May 25, 2016 #4

    mfb

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    Forget the 20 N force, you used that to calculate torque, it is not necessary any more. Yes you have the force from both sides, but you also want to apply it on both sides at the wire, so that doesn't matter.

    You have a torque of 3 Nm and you use it to apply a force at a distance of 1.5 cm. What is the force?
     
  6. May 25, 2016 #5
    the force is 200N but I dont understand why thats how it is done
     
  7. May 25, 2016 #6

    mfb

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    Because M=F*l works.

    Alternatively, consider the lever arms. They have a length ratio of 10:1, so force has a ratio of 1:10. You can apply a force of 200 N on the wire from both sides.
     
  8. May 25, 2016 #7
    hmm O am confused :/ what if i did it like this:

    20* 0,015=F*0,15

    This force is not 200N
     
  9. May 25, 2016 #8

    SammyS

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    It looks like 200N to me !
     
  10. May 25, 2016 #9
    it is 2N
     
  11. May 25, 2016 #10

    SammyS

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    I didn't look closely enough.

    You switched things up compared to post #1.
     
  12. May 25, 2016 #11
    yes thats the thing. why don't I get 200 N in my second attempt? I think the problem is is that I don't understand the scenario to begin with I just can't seem to grasp it :(...
     
  13. May 25, 2016 #12

    mfb

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    You swapped the distances, they should be at the other side of the equal sign, respectively.
     
  14. May 25, 2016 #13

    SammyS

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    upload_2016-5-25_14-40-9.png

    The pliers have two major pieces plus the pin as a pivot (fulcrum).

    Each handle has a jaw at the other end, all one piece.

    I have one such handle & jaw piece highlighted.

    The 20N force (upward) on that handle produces a counter-clockwise torque, 3Nm, on that piece. The object in the jaw must produce a clockwise torque of 3Nm to have rotational equilibrium.

    The torque pair on the other piece (handle &jaw) is totally independent from this. However, if the forces on the second piece don't cancel those on the first piece, the whole apparatus (pliers) will accelerate.
     

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  15. May 25, 2016 #14
    I get it now!!! thank you so much for your explanation :)
     
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