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Two string with one weight

  1. Mar 12, 2014 #1
    Just started with physics, I am not very good.

    1. The problem statement, all variables and given/known data
    A weight with the mass m is hanging in one thread (C) that is connected to two other threads (A and B). Thread A is completely horizontal, while thread B is forming a angle of 45° with the horizontal plane. All threads are connected in the way shown in the image. The force in thread A is 15N.
    My task is to figure out the force in thread B and the mass of the weight.
    g77OjOs.png

    2. Relevant equations



    3. The attempt at a solution
    I thought that a simple use of the Pythagorean theorem would give me the power in thread B; the triangle is 4 in height, 4 in weight. The A "contains" 15N of energy and is 6 units long, therefore i divided 15 by 6 and got 2.5.
    15/6=2.5
    (4*2.5)^2+(4*2.5)^2=200
    √(200)≈14.14N=B
    That is wrong, I am sure.

    I am not really sure how to solve this and I do not want the numerical answers to the questions, but rather tips and methods to solving the task.
    We were not really supposed to do this perticular assignment (our teacher told us it was "stuff we had not gone through yet") but I am curious to know how one should go about solving it.
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Mar 12, 2014 #2

    BvU

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    You are given the force in thread A. What do you know about the force(s?) on the point where A, B and C are joined togethrer, if this point isn't moving ?
     
  4. Mar 12, 2014 #3

    BvU

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    Oh, and: Hello ursus and welcome to PF!

    The grid is distracting you. A force on a string at one end is carried along to the other end, irrespective of the length of the string. You can check that with a spring (or a rubber band), a piece of (lightweight) string and a suitable weight.

    The nice thing about strings is that they can only exercise a force along the (taut) string. (if not "strung" then no force at all :smile:).

    Bottom line: the length of the various strings doesn't matter. The angles do. So if string A is pulling to the left and they tell you the force is 15 N, something else has to pull back with the same force -- otherwise there is no equilibrium and movement would result.

    In your case, wirde C can't do that, because all it does is pull in the vertical direction. Leaves only B. B is pulling up and to the right with equal forces, hence the 45 degree angle.

    Enough "hints" for now, go to work with this and either come with new questions, or with the solution ! Good luck.

    In the mean time, if you think something qualifies as "relevant equation", be sure to post it !
     
  5. Mar 13, 2014 #4
    Thanks alot. Did not expect that it would require almost no calculations. B is the only thing holding C from going to the left and since there is no motion, B must pull to the right with a force of 15 N. And also, since it is tilted at 45° then the force up must be equally as strong.
    The weight:
    15*9.82=1.47kg

    And the B thread:
    √(15^2+15^2)≈21.2 N
     
  6. Mar 13, 2014 #5

    BvU

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    Looks good, except the weight. Could you check that ?
     
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