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Tension in a String

  1. Sep 15, 2007 #1
    A 2.21 kg mass is suspended from a string which is pulled upward. The mass accelerates upwards with an acceleration of 3.50 m/s2. What is the tension in the string

    I don't have anything yet because I don't know where to start or what equation to use.
     
  2. jcsd
  3. Sep 15, 2007 #2
    it would help if you cheak your textbook and tells us what you are.eg:(pri.sec.jc.poly)
    (visit my blog!)
     
  4. Sep 16, 2007 #3

    Doc Al

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    Identify the forces acting on the mass (draw a diagram with forces labeled) and apply Newton's 2nd law.
     
  5. Sep 16, 2007 #4

    arildno

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    Hint:
    First choose whether "upwards" should be the positive direction or negative.
    Then, according to this choice, determine the sign of the known force(s) and accelerations.

    The correct sign for the unknown force will then appear out of thin air when you have formulated the problem correctly.
     
  6. Sep 16, 2007 #5
    I am using f=ma and a I am putting as positive is that not right?
     
  7. Sep 16, 2007 #6

    arildno

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    To say that a is positive is perfectly valid. :smile:

    Now, f is the sum of forces acting upon your mass.
    What individual forces is f the sum of?
     
  8. Sep 16, 2007 #7

    Doc Al

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    Nothing wrong with that, using up = +. What forces act on the mass?
     
  9. Sep 16, 2007 #8
    the tension and gravity
     
  10. Sep 16, 2007 #9

    Doc Al

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    Good. Which way do they act? What's the force of gravity equal to? Call the tension "T" and add them up.
     
  11. Sep 16, 2007 #10
    gravity acts down (g=9.8) and T acts up.
     
  12. Sep 16, 2007 #11

    arildno

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    So, weight is a "minus" force, agreed?

    Set up Newton's second law!
     
  13. Sep 16, 2007 #12

    Doc Al

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    Good. Set up an equation per Newton's 2nd law and solve for T.
     
  14. Sep 16, 2007 #13
    Fnet=ma
    T-g=ma
    T=(ma)+g
    is that right?
     
  15. Sep 16, 2007 #14

    Doc Al

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    Almost. g is the acceleration due to gravity, but weight is mg.
     
  16. Sep 16, 2007 #15
    Okay so its weight that is the force acting down? So T=(ma) + w...I am sorry this question is simple and I am just not getting it and I am so frustrated right now.
     
  17. Sep 16, 2007 #16

    Doc Al

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    You've almost got it. Keep going!
     
  18. Sep 16, 2007 #17
    I am getting an answer of 29.4151 N...
     
  19. Sep 16, 2007 #18

    Doc Al

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    Sounds about right. Round off your answer to a reasonable number of significant figures.
     
  20. Sep 16, 2007 #19
    Thank you so much! Another question though for a problem like this...

    A sphere of mass of 1.39 kg is accelerated upwards by a string to which the sphere is attached. Its speed increases from 3.01 m/s to 4.81 m/s in a time of 2.28 s. Calculate the tension in the string, assuming that the tension remains constant during that time.

    How do you incorporate the speed?
     
  21. Sep 16, 2007 #20
    Would you use Vf = Vo + at to find the acceleration?
     
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