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Homework Help: Calculating Moment

  1. Oct 7, 2007 #1
    1. It is known that the connecting rod AB exerts on the crank BC a 2.5kN force directed down and to the left along the center line of AB. Determine the moment of that force about C.

    http://img267.imageshack.us/img267/6822/fic03p013hb0.png [Broken]




    2. cross product



    3.
    I found the angle using tan-1 = 144/42 = 73.7

    Fx = -2.5cos73.7 = -0.702
    Fy = -2.5sin73.7 = -2.40

    F = (-0.702i -2.40j + 0k)
    CB = (-42i -56j + 0k)


    F x CB= (-0.702i -2.40j + 0k) x (-42i -56j + 0k)
    = -61.5 kN mm

    does that seem right??????????????????????????

     
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Oct 7, 2007 #2
    that first was the vector approach but a scalar approach is


    y and x compents then multiply by distance and just add them

    Fx = 2.5 x cos 73.7 = 0.702
    Fy = 2.5 x sin 73.7 = 2.40

    Clockwise = -
    Counterclockwise = +

    Mx = -0.702 x 56
    My = 2.40 x 42

    M = Mx + My = 61.5 kN mm


    can someone make sure im interpreting the question accurately ! :)

    thanks!
     
  4. Oct 7, 2007 #3
    Anyone plzzzzzzzz
     
  5. Oct 8, 2007 #4
    no knows?
     
  6. Oct 8, 2007 #5

    Astronuc

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    Staff Emeritus
    Science Advisor

    I don't have a lot of time at the moment, but 73.7° is the angle of arm AB with respect to horizontal. Are you sure. The 2.5 kN acts through AB at angle of 16.3° with respect to vertical.

    To find the moment on BC, one wants the force component from AB normal to BC.
     
  7. Oct 8, 2007 #6
    hmmm huh? so what i did was wrong?...

    even if i use 16.3 basically the y component of the force AB is going to be

    -2.5cos16.3= which is what i found -2.40
    and x component
    -2.5sin16.3= -0.702

    these are the components of AB if i want moment about C i can take a vector from C to B and cross it with AB right?

    AB x CB = 61.5 kN mm
     
  8. Oct 8, 2007 #7
    http://img229.imageshack.us/img229/4462/47859852fi8.jpg [Broken]
     
    Last edited by a moderator: May 3, 2017
  9. Oct 8, 2007 #8
    can someone atleast explain properly what im doing wrong if im doing anything wrong...
     
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