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: Trouble with a classic statics problem

  1. Sep 6, 2010 #1
    1. The problem statement, all variables and given/known data
    You are given three weights and 3 pulleys. Each mass hangs below a single pulley, and the pulleys are all connected by strings. The middle weight is 3 kg, the other two are each 2 kg. There are two unknown angles at the central pulley, with respect to the horizontal such that the lines connecting to the other pulleys would form the hypotenuse of a right triangle.

    Essentially it's this: http://forums.xkcd.com/viewtopic.php?f=18&t=45987 only the angles are unknown, and the masses are changed to what I have listed above.

    I have to find the angles if the system is at static equilibrium.

    2. Relevant equations
    None


    3. The attempt at a solution
    I tried breaking it down into force components, but something isn't right.

    I figured the tensions would be 2g, 2g, and 3g all told, and that I'd need the components for the "v" shaped lines. I ended up with 2gcos(theta)-3gcos(theta)+3gcos(theta)-2gcos(theta) for the sum of the x components, and 4gsin(theta)-3gsin(theta) for the y components, but this does not work out correctly. I am now completely lost, and have no idea how to properly set up the force diagram to solve the equation.
     
  2. jcsd
  3. Sep 6, 2010 #2
    I solved the problem. I remembered that tension in the system causes acceleration in a given direction if the force (in this case, just the mass) of the system is greater (or lesser) than the tension. So I found the equilibrium for that.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook