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: Free-hanging string with 4 attached masses!

  1. Sep 16, 2013 #1
    1. The problem statement, all variables and given/known data
    look at "oppgave 5".

    In the problem, you have a massless string with 4 uniform masses attached symmetrically, and you are supposed to show that the implicit equation for x (and x = cos(alpha)) is correct. Each of the masses are L/5 apart and the distance between the two ends of the string is D.

    3. The attempt at a solution

    I should be getting 5 equations for the 5 unkowns, but I am only getting 4 (the last 4 equations written on the paper)..

    Last edited: Sep 16, 2013
  2. jcsd
  3. Sep 16, 2013 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    This problem would make a lot more sense if I read Norwegian.
  4. Sep 16, 2013 #3
    Look at the picture in the pdf: You see a massless string weighed down by 4 attached point-masses. Each of the mass is a distance L/5 apart from each-other, where L = total length of the string. D= distance between the two ends of the string.

    Next you can see two angles: alpha and beta. The point with this problem is to show that the implicit equation for cosine of alpha written in the assignment,where x = cos(a), is correct.

    Please tell me if there is something more you do not understand! I really need help with this
  5. Sep 16, 2013 #4
    It is more convenient to simplify your notation by calling the tensions T and S.
  6. Sep 16, 2013 #5
    Then use your D equation to get cosβ in terms of x where x = cosα.
  7. Sep 16, 2013 #6
    I think that you do not need to use S[itex]_{3}[/itex].
  8. Sep 16, 2013 #7
    You are probably right, but if I removed [tex]S_3[/tex] I would have 4 unkowns and 3 equations.. I don't understand how I can get my last equation?

    Also sorry about the notation, but I took it from the assignment.
  9. Sep 16, 2013 #8
    You have FOUR equations.

    Two from vertical equilibrium and one from horizontal equilibirum if you do not use the third tension. But you have also your D equation from which to get cosβ in terms of x where x = cosα.
  10. Sep 16, 2013 #9
    You mean those involving G? But G is also an unknown, because I don't know the mass. Do you think G would disappear if I inserted the equations into the one for D?

    thanks for all help :)
  11. Sep 16, 2013 #10
    OK after a fresh look at the problem I see you were completely right. thanks !!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted