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: Support forces (statics)

  1. Oct 30, 2012 #1
    I've been trying to figure this out for 4 days now. I'm completely stumped. I've attached a picture of the diagram: http://postimage.org/image/t3d5i0l1v/

    I found the force in F:
    F = 25.04 lb ---> 16.92i + 18.46j
    Now I need to find the support forces at A and B.

    I'm not sure if this is right but I came up with the following equations:

    I used the angle at the supports as 42°.

    ƩFx=0= -Bx*cos42 - Ax*cos42 +16.92
    ƩFy=0= - By*sin42 - Ay*sin42 +18.46

    Then I thought about taking the moment about point A:

    ƩMa=0=Fx*2 + Fy*9 - Bx*cos42 *(9) - Bysin42 *(8)

    Then the moment about the other points F and B. A the end I got the following equations.
    ƩFx (Eq. 1): (0.743)Ax + (0.743)Bx = 16.9
    ƩFy (Eq. 2): (0.670)Ay + (0.743)By = 18.5
    ƩMa (Eq. 3): (6.7)Bx + (5.63)By = -200
    ƩMb (Eq. 4): (6.7)Ax - (5.63)Ay = 100

    Now when I try to solve these equations my unknowns cancel out!! What’s the deal!!!!!??
    Last edited: Oct 30, 2012
  2. jcsd
  3. Oct 30, 2012 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    It's not clear why F = 25.04 lbs. What was the original problem statement?
  4. Oct 30, 2012 #3
    I'm sorry I didn't clarify the force, F.

    The given information was: "The force F creates a 200 lb moment about point A and a 100 lb moment about point B. Calculate the force F." I created the equations to find the force using the geometry and points given in the diagram.

    The question did not ask about the support forces but I want to figure them out just for knowledge.
  5. Oct 30, 2012 #4
    I'm not sure why you are multiplying Bx, By, Ax, Ay by angles. The component of B in the x direction is Bx. The component of B in the y direction is By.

    If B was your vector and B was the magnitude of the vector B then I could see Bx = B cos 42, By = B sin 42, etc. But you notation is shorthand for B = Bx i + By j
  6. Oct 30, 2012 #5


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Please confirm coordinates of the point of application of F. The sketch shows (4,3) but it scales as (3,4).

    Also, Bx and By are not trig related as stated, since member AB is not a 2 force member.
  7. Oct 30, 2012 #6
    The coordinates for F are (4,3).
  8. Oct 31, 2012 #7
    You're right aralbrec.....I usually use them as a reference for which axis I'm working with. I completely forgot about that! Thanks for the reminder!
  9. Oct 31, 2012 #8


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    from the vertical
    your error here has already been addressed
    Since the support at B is a roller, it can only support forces perpendicular to AB; so in your above equation, take note of this
    make corrections....
  10. Nov 7, 2012 #9
    OMG....I can't believe I didn't see that. Thank you friend. I'm done now!!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook