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How do I find the tension in a cable in an elevator cab?

  1. Jan 31, 2016 #1
    1. The problem statement, all variables and given/known data
    "An elevator cab and its load have a combined mass of 1600 kg. Find the tension in the supporting cable when the cab, originally moving downward at 12 m/s, is brought to rest with constant acceleration in a distance of 42 m."

    ##m = 1600 kg##
    ##v = 12 m/s##
    ##d = 42 m##

    2. Relevant equations
    ##t = d/v##
    Answer: ##T = 1.8 * 10^4 N##

    3. The attempt at a solution
    ##t = (42 m)/(12 m/s) = 3.5 s##
    ##v/s = (12 m/s)/(3.5 s) = 3.43 m/s^2##
    ##T = (1600 kg)(9.8 m/s^2 - 3.43 m/s^2) = 10192 N ≠ 18000 N##

    I honestly have no idea how to tackle this problem, am just making guesses, and find it difficult to be doing this everyday as a student engineer. It's supposed to be so simple, yet I can't bloody do this. I'm doubting my ability to enter the engineering field if I can't do a simple problem such as this.
     
  2. jcsd
  3. Jan 31, 2016 #2

    SteamKing

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    You've just thrown some calculations together at the end of your post.

    There is a SUVAT equation which combines initial velocity, final velocity, distance traveled, and acceleration. Have you tried using that to find the acceleration a?
     
  4. Jan 31, 2016 #3
    The textbook did not give me any equations to work with, so I didn't even know to use them.
     
  5. Jan 31, 2016 #4

    SteamKing

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    You've got a computer. There's a wealth of knowledge at your fingertips. Google "SUVAT equations" and you can find what you need.

    P.S.: What kind of textbook doesn't have equations in it? Throw that thing away. It's defective.
     
  6. Jan 31, 2016 #5
    https://en.wikipedia.org/wiki/Equat...translational_acceleration_in_a_straight_line

    From that link...
    ##v^2 = u^2 + 2as##
    ##0 = (12 m/s)^2 + 2a(42 m)##
    ##a = -(12 m/s)^2/2(42 m)##
    ##a = -1.7143 m/s^2##
    ##F = (a + g)(1600 kg) = (-1.7143 m/s^2 - 9.8 m/s^2)(1600 kg) = -18,423 N##
    ##T = -F = 18,423 N##

    Thank you for saying this. I thought it was just me. I made a whole topic voicing my complaints on it, and I was convinced to keep using it, until now.
     
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