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!

Block and strut (Equilibrium)

  1. Nov 23, 2009 #1
    https://wug-s.physics.uiuc.edu/cgi/courses/shell/common/showme.pl?cc/DuPage/phys2111/fall/homework/Ch-12-Equilibrium/block-strut/block-strut-eq.gif [Broken][/URL]

    Sorry, forgot to add the picture...

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

    The system shown to the right is in equilibrium. The steel block has a mass m1 = 248 kg and the uniform rigid aluminum strut has a mass m2 = 47 kg. The strut is hinged so that it can pivot freely about it's bottom end. The angle between the left wire and the ground is Θ = 32o and the angle between the strut and the ground is φ = 49o

    2. Relevant equations

    torque = rFsin(theta)

    3. The attempt at a solution

    a) What is the tension in the vertical wire that holds the steel block? 2432.88 N
    b) What is the tension in the left angled wire?

    torque(hinge) = 0 = m(block)Lcos(49) + 0.5(m(strut)Lcos(49) - T2Lcos(49-32)
    I got T2 = 1827.197

    That seems low for where the pivot is.
    Any idea were I screwed up? Thanks!
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Nov 23, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    We can't help you because we can't see "the system shown to the right."
  4. Nov 24, 2009 #3
    Sorry about that...forgot to add the pic.
  5. Nov 24, 2009 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    You need sin(49-32) in your expression, not cos(49-32). Think of it this way, if Θ = Φ, i.e. the cable is pulling in along the direction of the strut, the torque due to the tension should be zero. This is the case if you use the sine and not the cosine.

    Also, be sure to multiply all masses by g to get the weights. Your expression omits g.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook