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Homework Help: Simple, resultant force problem but stuck.

  1. Feb 7, 2010 #1
    1. The problem statement, all variables and given/known data
    Determine the height h above the base B at which the resultant of the three forces acts.
    prob280.jpg

    2. Relevant equations
    Rx=F1,x + F2,x + F3,x

    3. The attempt at a solution
    Rx=(-300)+650+(-250)
    Rx=100

    but where?! The picture shows the diameter decreasing a tad just under the 650 force, does that matter?

    Anyways, am I suppose to use moments/couples? Dont moments/couples have to be equal in magnitude, opposite/parallel?

    Thanks,

    Frozenguy
     
    Last edited: Feb 7, 2010
  2. jcsd
  3. Feb 7, 2010 #2
    Hi there, Now I remember only a little bit from my statics class, but you did find fx correctly. Now for the height. Moments are the correct way to do it, I would suggest to find the total for all moments and see where you can go from there. Once you get that, think of the moment formula (M=distance x force). And keep in mind your trying to solve for distance.
     
  4. Feb 7, 2010 #3
    Oh ok!

    So I add up M1, M2, and M3 (defining base B as the point of rotation) and that will be M0 because of that guys law (I'm terrible, I need to look up his name). Then M0 divided by the magnitude of R will be the distance d.
     
  5. Feb 7, 2010 #4
    It should be. Do you have an answer for it in the book?
    I got an answer that seems reasonable. What's yours?
     
  6. Feb 7, 2010 #5
    I got that h would be equal to 45" above base B..

    It's an even problem, which the book lacks answers for :/

    I think this answer makes sense.. so is it saying that at 45" above B, there is 100lb of force acting towards the right? Or is this just to gauge the relationship between the forces?


    Thanks so much for your help btw.
     
  7. Feb 7, 2010 #6
    That is exactly what it is saying, your resultant force is 100lb above B 45". This moment equals all the other moments acting on the beam from the other forces. Just like summing up the forces, your summing up the moments.

    No problem for the help. I've been helped many times on here before, I figured I'd give back to the community.
     
  8. Feb 12, 2010 #7
    The beauty of this is that you can take moments about ANY point
     
  9. Mar 13, 2010 #8
    what do you mean any point? are you referring to verignons lawl?
     
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