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Mechanics Question III

  1. Apr 20, 2006 #1
    Im back :rofl:

    A large log AB is 6 m long. It rests in a horizontal position on two smooth supports C and D, where AC = 1 m and BD = 1 m, as shown in Figure . David needs an estimate of the weight of the log, but the log is too heavy to lift off both supports. When David applies a force of magnitude 1500 N vertically upwards to the log at A, the log is about to tilt about D.
    (a) State the value of the reaction on the log at C for this case.
    ... 0....
    David initially models the log as uniform rod. Using this model,

    (b) estimate the weight of the log
    [ I got 3750 N for this]

    The shape of the log convinces David that his initial modelling assumption is too simple. He removes the force at A and applies a force acting vertically upwards at B. He finds that the log is about to tilt about C when this force has magnitude 1000 N. David now models the log as a non-uniform rod, with the distance of the centre of mass of the log from C as x metres. Using this model, find

    (c) a new estimate for the weight of the log,

    im absolutely stuck here... the way ive laid it out is;
    Moments at D => 1500x5 =W(4-x)
    Moments at C => 1000x5 =????
    then... ??? :confused:

    I dont know what to do...

    (d) the value of x.

    Any help would be appreciated. :)
  2. jcsd
  3. Apr 20, 2006 #2
    You basically have it.

    Your equation 1500*5-W(4-x)=0 is right
    and the other is just 1000*5-W*x=0 (if x is measured to the right of C)

    Now you need to solve the 2 equations and 2 unknowns.
  4. Apr 21, 2006 #3
    :cry: im not getting the answer...
  5. Apr 21, 2006 #4


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    Gold Member

    What answer are you getting? And can you show your working please...

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