1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Simple truss question I can't seem to get right

  1. Jul 2, 2016 #1
    1. The problem statement, all variables and given/known data


    ignore all the other writing!

    3. The attempt at a solution
    I can't tell where I'm making a mistake. Probably a massive oversight somewhere but I don't know where!

    First find the reaction forces at the pin and roller by treating everything as a rigid body. (I am now aware this isn't necessary since I can start at joint D since one of the equations has 1 unknown, but this method is still bugging me)

    sum F_x = 0
    R_Dx + R_Ax - 5000sin(15) = 0 (equation 1)

    sum F_y = 0
    -R_Ay - 5000cos(15) = 0
    so R_Ay = -4829 N

    sum Moments about point A = 0
    (2*R_Dx) - (5*5000sin(15)) = 0
    so R_Dx = 3235 N

    so plugging R_Dx into equation 1 gives
    R_Ax = -1941 N

    starting at joint D

    sum F_x = 0

    R_Dx + BD = 0
    so BD = -R_Dx = -3235 N

    but the answer says it is 12.89 kN compression!
    I can't continue because everything else will be wrong.

    What am I screwing up on?
  2. jcsd
  3. Jul 2, 2016 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    It appears you have accounted for the moments about A due to the horizontal component of the load at C.

    However, additional moment at A will be created due to the vertical component of the load at C, since this component is offset horizontally from the line A-D.
  4. Jul 2, 2016 #3
    Dang, can't believe I missed that.

    Thanks for the reply!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted