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Tension problem. Please Help

  1. Feb 5, 2017 #1
    1. The problem statement, all variables and given/known data
    Two objects (2kg and 3kg) are attached by a rope which can withstand maximum tension force of 350N. Forces acting on objects change with time (F=k*t) where k=10N/s.
    After how many seconds will rope break. Solution is 25s.
    Capture.PNG

    3. Attempt at the solution:

    I have tried everything but I can't figure it out. Only thing I was able to calculate was acceleration , a=FT/m1+m2, a=70m/s2 but I dont think it's correct.

    P.S. Sorry for my english, its not my native language.
     
  2. jcsd
  3. Feb 5, 2017 #2

    haruspex

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    I do not understand the problem description/diagram.
    The diagram shows three forces (F). The middle and right ones have the same subscript, either 2 or z, but the left one looks different. The left and right ones have marked directions, left and right respectively, but the middle one does not.
    Which are actual applied forces, and are they equal in magnitude at all times?
     
  4. Feb 5, 2017 #3
    Middle one is Fz or tension force, left one is F1, right one is F2. In the middle there is an x sign which is just representing breaking I guess.
     
  5. Feb 5, 2017 #4

    haruspex

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    Are F1 and F2 both =kt, same k? If so, why would there be any acceleration?
     
  6. Feb 5, 2017 #5
    Yes for both forces =kt. I have no idea really how to start. I know how to work out standard tension problems but this one is really difficult.
     
  7. Feb 5, 2017 #6
    Can you verify that the problem is stated correctly. Something doesn't seem right. At 25 seconds, if F = kt, where k = 10, the force(s) would only be 250 N. How could the rope break with 250 N of force?

    P.S. Welcome to Physics Forums.
    P.S.S. I think your english is very good.
     
  8. Feb 5, 2017 #7

    haruspex

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    If the two applied forces are equal and opposite then there is no acceleration. It will take 35 s to reach 350N.
     
  9. Feb 5, 2017 #8

    haruspex

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    pps. (Smiley)
     
  10. Feb 5, 2017 #9

    haruspex

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    I can get the desired answer if F1=kt and F2=2kt.
     
  11. Feb 6, 2017 #10
    I'll try to translate the problem one more time.

    Two objects (2kg and 3kg) are attached by a rope which can withstand maximum tension force of 350N. (I hope this part is clear)
    Forces are acting on objects and their intensity change with time F=kt, where k is constant (10 N/s). Calculate the length of time from the beginning of the movement until rope breaks.
     
  12. Feb 6, 2017 #11

    haruspex

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    As I indicated, it cannot be that the two forces are supposed to be the same. There would be no acceleration, so it becomes a trivial problem with a different answer.
     
  13. Feb 6, 2017 #12
    Are there two forces or three forces? Is that force shown between the two masses an externally applied force, or is it just a confusing way of showing that there is tension in the rope.
     
  14. Feb 6, 2017 #13
    There 2 forces and in the middle is tension force.

    Is there any way you can get 25s ?
     
  15. Feb 6, 2017 #14

    gneill

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    There are two ways already mentioned:
    1) There is an error in the problem statement where the maximum tension is actually 250 N instead of 350 N, and
    2) There is an error in the problem statement where F1 is not equal to F2 but rather F2 = 2F1 where F1 = kt.

    Otherwise the problem does not have a solution that matches what is given.
     
  16. Feb 6, 2017 #15
    In that case thanks for help. I guess there's just something wrong with the problem then.
     
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