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Homework Help: Dynamics: Newton's Second Law_Tension_1

  1. Jan 8, 2015 #1
    1. The problem statement, all variables and given/known data

    *Note: I'm really stuck in this part of Newton's 2nd Law so I may be asking a few more questions after this to practice for our upcoming examinations.

    A cable supporting a 2125kg elevator has a maximum strength of 21750N. What maximum upward acceleration can it give without breaking?

    2. Relevant equations

    1. Forcevertical= Forcetension + Forcegravity
    2. Force= (mass)(acceleration)

    3. The attempt at a solution
    What I did is:
    Forcevertical=21750N + [(-9.8m/s2)(2125kg)]

    Then after solving the first part, i used the 2nd equation then did this:
    925N = (2125N)(a)
    a = 0.435 m/s2

    I'm not quite sure if I'm right in this problem.

    Thank you in advance.
  2. jcsd
  3. Jan 8, 2015 #2


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    Staff: Mentor

    Method looks right. I've misplaced my sliderule at the moment, so haven't checked your multiplication answer. :)
  4. Jan 8, 2015 #3
    The calculations are correct. You did nothing wrong that I ca tell.
  5. Jan 8, 2015 #4
    thank you for checking it! but i dont know why my classmates keep on proving me wrong though
  6. Jan 8, 2015 #5
    Well the question is basically asking how fast you can move the elevator up with respect to the mass of the elevator. So the answer makes sense. If they're arguing with it I'm curious as to what they're doing.
  7. Jan 8, 2015 #6
    Thank you! I thought i did it wrong but it was right all along. :)
  8. Jan 8, 2015 #7
    Well, even I seem to get this answer only.
    Your answer seems right to me.
  9. Jan 8, 2015 #8
    I think you are right ,,
    I'm wondering about what your friend said ,,,
  10. Jan 8, 2015 #9


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    Science Advisor
    Homework Helper
    Gold Member

    You don't say what answer others get, so it's hard to tell. An important thing to notice is that the calculation involves taking the difference of two numbers that are close together. This creates the risk of error magnification. E.g. if you take g as 10m/s2 you get only half the answer.
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