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Calculating the moment about an axis?

  1. Jan 30, 2011 #1
    1. The problem statement, all variables and given/known data

    Hi friends, I have uploaded the problem and picture at the link below. Please take a look.

    http://img64.imageshack.us/img64/4753/sam0247i.jpg [Broken]

    2. Relevant equations

    F(vector) = F(cos[tex]\alpha[/tex]i + cos[tex]\beta[/tex]j + cos[tex]\gamma[/tex]k)
    Ma = u_a(vector) [tex]\bullet[/tex] M_o (vector) = u_a [tex]\bullet[/tex] (r x F)

    [tex]\bullet[/tex] is supposed to denote dot product. Sorry my latex sucks.

    3. The attempt at a solution

    Okay so for this problem, I currently have:

    F = 30 (cos60i + cos60j + cos45k) = (15i + 15j + 21.2k)

    Now I know I need to find the unit vector u, and position vector r. How do I find these? From the examples I've done, I've kinda known what to use for vectors u and r, but this example is different from the ones that I've worked on previously.

    The force for this one is extending right from origin, whereas the problems I've previously worked on had forces away from origin.

    So how would I find the unit and position vectors for this problem? And how can I determine the coordinate direction angles to produce the max moment and the max moment itself? Is there some sort of equation?

    Thanks for your help and time. I'm not looking for answers here, but just something to get me rolling cause I'm stuck and I really want to figure this out but don't know how!
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Jan 31, 2011 #2


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    hi mneox! :smile:

    (have an alpha: α and a beta: β and a dot: · and try using the X2 icon just above the Reply box :wink:)
    u is the unit vector along the pipe (so that's j)

    r is any vector from the line of the force to the pipe …

    the reason you can use any vector is that you're only interested in (r x Fu, and if you increase r by a multiple of F or of u, it makes no difference :wink:
  4. Jan 31, 2011 #3
    Thanks for the reply tiny-tim! I think I'm grasping it now, but what do I do about the maximum moment part? How would I go about doing this?

    ps thanks for the nifty copy and paste lol
  5. Jan 31, 2011 #4


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    you need to maximise the value of u.(r x F), keeping u r and the magnitude of F constant, and varing only the direction of F

    which direction will do that? :wink:
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