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: Tension on ropes

  1. Apr 20, 2007 #1
    1. The problem statement, all variables and given/known data
    A 1100 kg steel beam is supported by two ropes. Each rope has a maximum sustained tension of 6200 N.

    Then it shows a diagram of two ropes holding a steal beam at the center, both are angled out. The rope on the left is angled away at 20 deg from y axis. The rope on the right is angled 30 deg from y axis.

    2. Relevant equations
    Fnet = ma

    3. The attempt at a solution
    So basically I used substitution...
    R1 = rope 1 (left)
    R2 = rope 2 (right)
    E = summation
    F = Force

    E(Fx) = MAx = 0

    R2*sin(30) - R1*sin(20) = 0
    R2*sin(30) = R1*sin(20)
    R2 = R1*[sin(20)/sin(30)]

    Plug that into sum of forces in y direction...

    E(Fy) = MAy = 0

    R1*cos(20) + R2*cos(30) - Mg = 0
    R1*cos(20) + R2*cos(30) = Mg = 10791 N
    plug in R2...
    R1*[cos(20) + sin(20)/sin(30)] = 10791
    R1 = 6645.96 N

    Now I did it like this.. plugged that back into the original and got 4546.11 N for R2. However it was wrong. So I thought okay since the max tension is supposedly 6200 I'll use that as R1 and got 4241.05 for R2. Wrong again. Any ideas?
  2. jcsd
  3. Apr 20, 2007 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Redo your plugging in of R2; looks like you dropped the cos(30).
  4. Apr 20, 2007 #3
    Not sure where you're getting the cos... both are sine since the x vector is opposite of the angle.. making it sine. Right?
  5. Apr 20, 2007 #4

    Doc Al

    User Avatar

    Staff: Mentor

    The cosine you dropped is the one right here:
  6. Apr 20, 2007 #5
    oh wow what a stupid mistake, thanks! it works now
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook