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: [Physics Equlibrium Question] A spider hanging from two silk threads

  1. Feb 2, 2014 #1
    [Physics Equlibrium Question] ***[SOLVED]***


    Hey, I haven't used a forum before, but out of desperation to find the solution/hint to this problem; google brought me here. I would really appreciate any help, as I am I not getting the correct answer. Thank you.


    1. The problem statement, all variables and given/known data

    In the figure below, a spider is resting after starting to spin its web. The gravitational force on the spider is 0.170 N on the junction of the three strands of silk. The junction is supported by different tension forces in the two strands above it so that the resultant force on the junction is zero. The two sloping strands are perpendicular, and we have chosen the x and y directions to be along them. The tension Tx is 0.104 N.

    Find the Find the tension Ty, find the angle the x axis makes with the horizontal and Find the angle the y axis makes with the horizontal.

    I attached a picture of the diagram and my FBD of the situation.

    2. Relevant equations

    Θ1+Θ2=90° (Equation 1)
    ∑Fx=0.104sin(Θ1)-[(T2)sin(Θ2)]=0 (Equation 2)
    ∑Fy=0.104cos(Θ1)-[(T2)cos(Θ2)]-0.15=0 (Equation 3)

    3. The attempt at a solution


    Θ1=Θ2-90° (Rearranged equation 1)

    0.127sin(Θ2-90°)-[(T2)sin(Θ2)]=0 (Equation 2, subbed rearranged equation 1 into Θ1)

    T2= [0.104sin90°-0.104sin(Θ2))/sin(Θ2)] (Rearranged equation 2 to solve for T2)
    T2= 0.104sin90°-0.104=0 (I GET ZERO HERE?)
    --NOTE:the below is not edited for the correct value of T1=0.104N because the above is incorrect for some reason now.. I don't know how to proceed now.

    0.127cos(Θ1)-[(T2)cos(Θ2)]-0.15=0 (Equation 3)
    0.127cos(Θ1)-[(-0.12353)cos(Θ2)]-0.15=0 (subbed new T2 value into Equation 3)

    0.127cos90°-0.127cos(Θ2)+(-0.1235)cos(Θ2)-0.15=0 (subbing in known values to find Θ2)
    Θ2=95.28° (this angle does not make sense )


    Attached Files:

    Last edited: Feb 2, 2014
  2. jcsd
  3. Feb 2, 2014 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    I think you are missing some information. Having the angle between the supporting threads is not enough information to solve the problem. You need to know the angle between one the threads, w.r.t. either the horizontal or vertical.

    The given tension in one of the threads is 0.104 N. It's not clear where 0.127 N comes from in your calculations.
  4. Feb 2, 2014 #3
    opps sorry I meant 0.104N instead of 0.127N and that is all the information the the question gave me. I'll input the correct values now.
  5. Feb 2, 2014 #4
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted