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Free Body Diagram Of a Situation

  1. Oct 29, 2011 #1
    1. The problem statement, all variables and given/known data
    Hello people,
    First of all, this is my first day in this forum and this post is my very first post. So, I am extremely sorry if I violate any rules here.
    I have to solve a problem involving frictions. I will solve the problem myself; I just need help with drawing the Free Body Diagram of it. Here is the problem:
    http://www.solutioninn.com/engineer...-w1-is-placed-across-the-channel-and-boy-of-w


    2. Relevant equations
    F = usN (us is the coefficient of static friction)


    3. The attempt at a solution
    I have drawn the Free Body Diagram; I would like to make sure if it's correct or not.
    Here's what I think is correct. Am I missing anything here?
    [PLAIN]https://fbcdn-sphotos-a.akamaihd.net/hphotos-ak-snc7/310450_10150377422927432_808377431_8363863_2056490143_n.jpg [Broken]
     

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    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Oct 30, 2011 #2

    I like Serena

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    Welcome to PF, Arka2334! :smile:

    In this FBD of the board combined with the boy, you need another force of friction at A.

    Btw, the friction at B should be in the opposite direction, since the board would tend to slide down, and the friction on the board opposes that movement.
     
  4. Oct 30, 2011 #3
    Ahh, thank you very much. Right, the Friction in point B DOES have to be in the opposite side; that was a silly mistake. I was just unsure if I need any force of friction in point A or not, since at that part, the board just seems to be resting on the surface. That was the main reason I asked this question actually.
    Anyway, thank you very much for your help. ^_^
     
  5. Oct 30, 2011 #4

    I like Serena

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    You're welcome! :smile:

    Btw, in an FBD you're supposed to have all the forces, even if some of them may turn out to be zero.
    If you're not sure a force is zero, you'd better include it - better safe than sorry.
    Only if there are no horizontal forces in play at all can you afford to leave the possible horizontal forces out.
     
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