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: Static Equillibrium

  1. Feb 8, 2010 #1
    Hi everyone, first time post at physics forums, I hope to help out some people with calc and I hope to discuss some physics topics too! But however I have a homework question in MECH 234.

    1. The problem statement, all variables and given/known data

    Find Tac + Tbc from the given picture:


    2. Relevant equations

    ΣFx = 0
    ΣFy = 0

    3. The attempt at a solution

    Well first, since 500N is being pulled on the x axis-

    ΣFx = 0 = Tac*sin(50)+Tbc*sin(30) - 500

    500 = Tac*sin(50)+Tbc*sin(30)

    The right side is the sum of all the Fx components and they must be equal to zero.

    ΣFy = 0 = Tac*cos(50)+Tbc*cos(30)

    The right side is the sum of all the Fy components and they must be equal to zero.

    When I try to solve the two equations (2 unkowns 2 equations) I just get crazy numbers. I do not know what mistake I could be making otherwise except the angles might be wrong. Or perhaps I should be using the law of cosines or sines?
  2. jcsd
  3. Feb 8, 2010 #2
    You may be making the mistake in the equation you have for the y dimension. You have that the sum of all y forces is zero, which is correct. However, you may have made your mistake by not setting Tac*cos(50) equal to Tbc*cos(30). Did you mistakingly say Tac*cos(50) equals -Tbc*cos(30)? Also, put up the numbers you got so it is easier to find where you went wrong.
  4. Feb 8, 2010 #3
    Well I solved for the equations:

    0 = Tac*cos(50)+Tbc*cos(30)
    500 = Tac*sin(50)+Tbc*sin(30)

    and got

    Tac ~ 1.98
    Tbc ~ -199

    I did not know that you need to set Tac*cos(50) equal to Tbc*cos(30).

    Why do they have to be equal? Is it because there is no y component in the 500lb force; therefore, both components need to be equal to each other for equilibrium?

    My answer for solving the system of equations-

    500 = Tac*sin(50)+Tbc*sin(30)
    Tac*cos(50) = Tbc*cos(30)

    Tbc= 326.3518 N
    Tac= 439.6926 N

    It is correct. And I would like to thank you in advance.
    Last edited: Feb 8, 2010
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook