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: Statics - Moment using scalar analysis

  1. Oct 21, 2009 #1
    1. The problem statement, all variables and given/known data


    2. Relevant equations

    I think that I need to find the moment of the force F about the line BC first.

    Then, using that moment, find the "projection" of [itex]M_{BC}[/itex] onto the X axis to find the answer. [itex]M_{BC}\sin \left(45^{o}\right)[/itex]

    However, I am not getting anything near the solution value.

    3. The attempt at a solution

    My only attempt at solving this that came close to correct was to use the distance from point B to F. That was obviously wrong (but yielded 20 lb - which was extremely close to the 20.2 given by the author.)

    (I know that is wrong...) As soon as I sat the problem down to go to the next one, I immediately realized my method was incorrect, but I have tried and tried to get a correct answer and nothing has yielded in 1.5 hrs, so I digress.

    I have a test this afternoon, so I cannot waste a lot of time on one that is holding me up. Two more chapters to review...

    Any hints will be great!

  2. jcsd
  3. Oct 25, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    The moment of a force about the x axis is [tex]F_zy + F_yz[/tex]. Since only F_z is acting, there is no F_y term, so you just have to compute the y distance (the perpendicular distance from the x axis to the line of action of the force). You need to watch your geometry and trig to get the corect y value which yields the correct answer of F = 20.2 N.m.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook