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Tensile force calculation

  1. Nov 9, 2015 #1
    This question is confusing me, trying to find an equation for "tensile force" has left me confused since ive found no equation for it just for tension, tensile stress and tensile strain, just wanting to make sure ive not missed anything or done it incorrectly

    1. The problem statement, all variables and given/known data

    At a building site, an iron girder of mass 420 kg is suspended from a crane
    by a steel cable. Assume that the cable has a circular cross-section of
    diameter 25 mm.

    Determine the tensile force in newtons on the cable in kN to 2 decimal
    places. (Take the acceleration due to gravity as 9.8 m s−2 and ignore the
    mass of the cable).

    2. Relevant equations
    i calculated it as tension assuming tensile force means tension


    T = mg x ma

    3. The attempt at a solution
    (420kg x 9.8ms2) x (420kg x 9.8ms2) = 8232 kg ms2

    which i then converted to newtons = 8232N (8.23Kn)
     
  2. jcsd
  3. Nov 9, 2015 #2
    This is not correct. The tension in the cable is just equal to the weight of the girder.
     
  4. Nov 9, 2015 #3
    what about the gravitational force ? also is tension the same as tensile force or are they 2 separate measurements ?
     
  5. Nov 9, 2015 #4
    The gravitational force is the same thing as the weight.

    Chet
     
  6. Nov 9, 2015 #5
    so basically the tensile force is 420N or 0.42Kn, are tension and tensile force 2 different measurements or the same ?
     
  7. Nov 9, 2015 #6

    SteamKing

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    It's not clear where you got this equation. BTW, it's meaningless.
    (420 × 9.8) × (420 × 9.8) = (420 × 9.8)2 ≠ 8232

    Also, what are the units here? Hint: they're not kg⋅ms2

    Remember, g = 9.8 ms-2, which is not the same as 9.8 ms2. Things like - signs and × signs are important.
     
  8. Nov 9, 2015 #7
    They are different terms for the same thing.

    Chet
     
  9. Nov 9, 2015 #8
    The formula I used was

    T = mg + ma

    T = tension, N, kg-m/s2

    m = mass, kg

    g = gravitational force, 9.8 m/s2

    a = acceleration, m/s2
     
  10. Nov 9, 2015 #9

    SteamKing

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    Using × in place of + suggests something else. × means multiplication. + means addition.

    The formula T = mg + ma should be used only when there is some other acceleration a acting on m besides gravity g.
     
  11. Nov 9, 2015 #10
    Sorry I didn't catch that I'd used x rather than +, I used + In my actual calculation.

    So using 9.8ms-2 and not applying gravity as the acceleration would give me

    (420kg x 9.8ms-2) + (420kg x 0) =

    Or since there is no acceleration not include it at all like this?

    (420kg x 9.8ms-2) + 420kg =
     
  12. Nov 9, 2015 #11

    SteamKing

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    According to this calculation, you're saying that 420 kg × 0 = 420. Is that some kinda Common Core math?
     
  13. Nov 9, 2015 #12
    Where did I say that? This is the new equation I've come up with following the advice from this thread
     
  14. Nov 9, 2015 #13
    Compare you final two equations in post #10.

    Chet
     
  15. Nov 9, 2015 #14
    Those are 2 different equations, the first is where I've left acceleration in and the second is where I've removed it leaving only mass
     
  16. Nov 9, 2015 #15
    In that case, the first equation is correct and the second equation is incorrect.
     
  17. Nov 9, 2015 #16

    SteamKing

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    Yeah, but force = mass × acceleration

    Setting acceleration to zero means the force = 0, by definition.

    You can't be careless with equations, math, or units. These will cost you points on exams and assignments when studying physics if you don't make sure they're all correct.
     
  18. Nov 9, 2015 #17
    This is why I'm here to gain a better understanding and learn

    So the correct equation would be

    (420kg x 9.8ms-2) + (420kg x 0) = 4.37kg m/s-2
     
  19. Nov 9, 2015 #18

    SteamKing

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    I'm not sure how you can multiply 420 kg by 9.8 m/s2 and wind up with 4.37 kg-m/s2. That doesn't make sense arithmetically.

    Also, kg m/s-2 is not the same as kg m/s2. Have you studied algebra yet? The laws of exponents?
     
  20. Nov 9, 2015 #19
    I haven't studied any algebra yet this is the first month of my distance learning degree, I multiplied it by 9.8m/s-2 [/SUP ] to get that answer
     
  21. Nov 9, 2015 #20

    SteamKing

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    Well, 420 x 9.8 ≠ 4.37

    You should be able to estimate what the answer should be without a calculator.
     
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