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Calculating tension on 3 wires/strings

  1. Jun 7, 2013 #1
    1. The problem statement, all variables and given/known data

    "a spider is resting after starting to spin its web. The gravitational force on the spider is 0.172 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 0.107 N."

    2. Relevant equations

    Tx+Ty+spider=0

    3. The attempt at a solution

    I attempted to make a triangle out of Tx and Ty but there are no values or angles to work with other than the 90 degrees b/w x and y. Any tips on how to solve for Ty would be much appreciated!
     
  2. jcsd
  3. Jun 7, 2013 #2

    TSny

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    Welcome to PF!

    Try looking at components of forces along the x direction and see if that tells you anything about the x-component of the gravitational force of the spider.

    [EDIT: Actually, if you draw a correct triangle of forces you can get the answer right away. Can you describe in more detail how you tried to construct your triangle? My comment above is another way to get to the answer using the method of components.]
     
    Last edited: Jun 7, 2013
  4. Jun 7, 2013 #3
    I don't think this is workable.
     
  5. Jun 7, 2013 #4
    alright, i made a right triangle with the x and y being the two sides and the given 90 degrees. it didn't get me anywhere earlier since I didn't have any values. I couldn't figure out how to calculate the values of the horizontal and vertical degrees of the strings, so I went with that triangle.

    as for the first part, by cos(90) is 0 and sin(90) is 1....so that leaves me back where i started, as well?
     
  6. Jun 7, 2013 #5

    TSny

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    There are 3 forces acting at the junction of the threads, right? One force slopes up along the x axis and you know the value of that force (Tx). Another force slopes up along the y axis and you are trying to find the value of that force. The third force acts vertically downward and you know the value of that force. So, you know 2 out of the 3 sides of the force triangle.
     
  7. Jun 7, 2013 #6
    it just clicked. that makes so much more sense than what i was doing. i really appreciate your help! thanks!
     
  8. Jun 8, 2013 #7
    Wait a minute. The problem states 3 strands and the spider.
     
  9. Jun 8, 2013 #8

    TSny

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    The wording of the problem isn't clear, but it seemed to me that the spider must be hanging from the end of the 3rd strand.
     
  10. Jun 8, 2013 #9
    I guess you have to assume the third strand has a tension of 0 then the problem is workable.
     
  11. Jun 8, 2013 #10

    TSny

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    Here's how I interpreted it.
     

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