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Homework Help: Min and max value of moment

  1. Mar 29, 2008 #1
    1. The problem statement, all variables and given/known data

    The 70N force acts on the end of the pipe at B. determine the angles theta between 0 and 180 of the force that will produce maximum and minimum moments about point A.

    2. Relevant equations

    FBD is attached.

    3. The attempt at a solution

    I made the following equation then took its derivative and put it equal to 0. The problem here is that I only get one value of theta. the one which gives the maximum value of the moment. there isn't any other value of tan coming withing 0-180. How do I get the minimum value of theta from this method. Wasn't I automatically supposed to get two value of theta from the first derivative and then I could I have gone further to second derivative to check if it was min or max. I am stuck.

    Equation :- [tex]M = (70cos\theta * 0.9) + (70sin\theta * 0.7)[/tex]
    Derivative :- [tex] = -63sin\theta + 49cos\theta [/tex]

    Attached Files:

  2. jcsd
  3. Mar 29, 2008 #2


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    Homework Helper

    Hi Altairs! :smile:

    (btw, I haven't seen the picture yet)

    erm … sometimes it's maths, and sometimes it's just looking at the reality.

    It often helps to draw a diagram (roughly).

    The graph of the moment should look like a hill.

    At 0º, the derivative is 49, which is positive; at 180º, it is -49, which is negative.

    So the moment is already increasing at 0º, and is still decreasing at 180º.

    So one of them may be a minimum … :smile:
  4. Mar 30, 2008 #3
    Sorry. Can't get it. Would be easier with a few equations.
  5. Mar 30, 2008 #4


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    Hi Altairs! :smile:

    (Still can't get the picture - I keep getting the please-log-in screen! :cry:)

    At 0º, the moment is 63; it goes up until tantheta = 7/9; then it goes down until, at 180º, it is -63.

    So the maximum is at tantheta = 7/9, them minimum is at 180º. :smile:
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