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: Forces in equilibrium. Tension, ring, pulley and lots of string [With Picture!]

  1. Feb 29, 2012 #1
    1. The problem statement, all variables and given/known data
    To make it easier on the people who will try to help me, I've scanned the problem:

    We are interested in finding the following variables:
    Angles a and c
    The tension between A and B (TAB)

    2. Relevant equations
    The pulley is a "perfect pulley" no friction
    The system is at equilibrium
    We consider on side A that the string is attached to the wall, or a hook, it's fixed.
    All three cords are attached to a ring situated at position B
    There is nothing between A and C, but we know there is 3 meters of distance between the two

    3. The attempt at a solution
    Alright so I decided to make a Free Body Diagram taking the ring as the subject. It goes like that:


    Considering the pulley, I guess we can assume that TBC is going to to be the same as FA.

    After that I decided to decompose each of the three forces, and since everything is at equilibrium, I've come with two equations (again in picture, gonna be easier):


    At that point I am stuck because I don't see any more equations involving those three unknown variables(a,c, TAB) I can do. So I have 3 unknown variables but only two equations.

    Could someone please help me out?
  2. jcsd
  3. Feb 29, 2012 #2


    User Avatar

    Staff: Mentor

    Have you considered using some of the trig laws? The law of cosines and the law of sines come to mind.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook