1. Not finding help here? Sign up for a free 30min 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!

Traffic light problem

  1. Dec 5, 2003 #1
    A traffic light weigh 235 Nt. It is supported by 2 wires cal the tension of each of the wires

    T1 has an angle of 21 degrees
    T2 has an angle of 38 degrees

    There is no sag in the light

    I have 2 equations

    equ 1) Fx = T2 cos 38 - T1 cos 21 = 0
    equ 2) Fy = T1 sin 21 + T2 sin 38 = 0

    I set T2 = to T1

    T2 = T1 (cos 21 / cos 38) = 1.2 T1

    then subsitutedthat into equatin 2

    T1 sin 21 + (1.2 T1)(sin 38) - 235 Nt = 0

    T1 = 214 N

    T2 = 1.2T1= 1.2(214)

    T2 = 257 N

    I think iI went about this in the right way i think

  2. jcsd
  3. Dec 5, 2003 #2

    Doc Al

    User Avatar

    Staff: Mentor

    I didn't check your arithmetic, but it looks good to me (assuming angles are with respect to the x-axis). A few comments:

    - I don't know what you mean by no sag: of course the wires "sag", that's why they are at an angle.
    - Your equation #2 has a typo (you left out the weight), but you correct it later on.
  4. Dec 5, 2003 #3


    User Avatar
    Staff Emeritus
    Science Advisor

    It doesn't look so good to me.

    What reason do you have for setting T1= T2?? It doesn't look to me like the tension in the two wires will be the same (the situation is not symmetric because the angles are different).

    Oh, wait! You didn't set T1= T2! You solved equation 1 for T1. That's a whole different matter.

    Also, as DocAl said, you second equation (for the vertical component of force) should be
    equ 2) Fy = T1 sin 21 + T2 sin 38 - 235 = 0
    so that the total vertical force is 0
    or equ 2)
    T1 sin 21 + T2 sin 38 = 235 so that the upward force of the wires balances the downward force of the light.

    You did manage to get the right answer!
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Traffic light problem