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Homework Help: Yet another statics problem involving several forces

  1. Jan 30, 2015 #1
    1. The problem statement, all variables and given/known data
    The body is connected to the ground with a moving joint at B, the body slides with no friction at A.

    2. Relevant equations
    ΣF=0 => ΣFx=0 & ΣFy=0

    3. The attempt at a solution
    First I made a drawing:

    At this point I started calculating:
    ΣFx=>P-R2-F1, so: P=R2+F1
    ΣFy=> R1+F2-mg=0, so: R1+F2=mg

    I'm quite confused when it comes to the moment around B, I know that Moment=force*distance, but how do I draw all the distance that I need?

    I thought about something like that:
    So, the distance between F1 and the point B is a, while the distance between F2 and B is b.
    Knowing this, I assume that the moment would look like that:

    Mb: P+mg-F2*b-F1*a-R2-R1=0
    now I replace P and mg according to the previous equations:
    Mb: R2+F1+R1+F2-F2*b-F1*a-R2-R1=0
    now the weird part is that I could reduce almost everything in the equation above, so that I would only be left with:
    Mb: b-a=0
    but it doesn't make any sense.

    I would appreciate any help, thank you in advance!
  2. jcsd
  3. Jan 30, 2015 #2
    Two things:
    1) the 'mg' force always acts from the center of mass of the object
    2) how exactly does the body slide? Seems to be some mistake there
  4. Jan 30, 2015 #3


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    I don't think they are saying it slides. I think the point is that there is no friction at "a" so that it could slide if "b" wasn't there. If there was friction at "a" (or if "a" was another mounting like that at "b") then I think you would find it hard to calculate how forces are shared between "a" and "b".
  5. Jan 30, 2015 #4


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  6. Jan 30, 2015 #5
    It says exactly that the body slides at A with no friction. I don't know exactly what they mean by that either. What do you think about the equations though?
  7. Jan 30, 2015 #6


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    what does it say about P and M?? In fact, what is the question as exactly worded?
  8. Jan 30, 2015 #7
    That is the problem, it doesn't say anything about either P or M. I assume that M is supposed to be Moment, while P is a force, but the whole question is "The body in the picture is connected to the ground with a movable joint and slides without friction at A. Simplify the drawing and find the forces". It doesn't even say how far in the calculations I'm supposed to go.
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