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Work done by the tension on a cable

  1. Jan 2, 2015 #1
    1. The problem statement, all variables and given/known data

    A crane lifts a 1250 kg load directly upward with a constant speed through a vertical
    distance of 8 m. How much work is done by the tension in the crane cable?

    2. Relevant equations

    Im thinking its the work equation but this is were i got stuck

    W = m(as)


    3. The attempt at a solution

    got confused right away on this one my thinking is

    m = 1250
    s = 8
    a = is this zero seeing as its constant?
     
  2. jcsd
  3. Jan 2, 2015 #2

    Doc Al

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    Staff: Mentor

    Where did you get that formula?

    What's the most basic definition of work?
     
  4. Jan 2, 2015 #3
    work is when a force moves its point of application through a distance .

    isnt that the same as W = Fs?
     
  5. Jan 2, 2015 #4

    Doc Al

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    That's the equation that you need. What's "F" in this problem?
     
  6. Jan 2, 2015 #5
    force? and that would be the mass x the acceleration

    but what do i say the acceleration is seeing as im told its constant?
     
  7. Jan 2, 2015 #6

    Doc Al

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    What force? (What force is doing the work in this problem?)

    No. Mass x acceleration gives you the net force on something.

    The acceleration would be zero.
     
  8. Jan 2, 2015 #7
    Im pretty confused now lol but I'm going to say its gravitational force bearing down ?
     
  9. Jan 2, 2015 #8
    Draw a FBD. Find the FT and then plug and chug into W = F x d
     
  10. Jan 2, 2015 #9

    Stephen Tashi

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    Science Advisor

    In order to be simple, physics exercises are often unrealistic. To lift a real object, a real crane probabily doesn't raise it at a constant velocity. In a real situation there is a fluctuating imbalance of forces, so there are small accelerations (and decelerations) of the load as it rises.

    In this problem, you are told the obejct is lifted with a constant velocity. Theoretically, to lift a object with a constant velocity, you could exert a force on it that exactly cancels gravity. (Tie a big helium ballon to it.) The object would be "levitated". Then you give the object a nudge and it would move upward at a constant velocity. There would be no net force on the object. One can still ask what work is done just by force that is canceling gravity. That work is non-zero.

    You have to pretend the crane is levitating the object and you have to ignore whatever work was done to start the object on its way up with a constant velocity.
     
  11. Jan 2, 2015 #10

    Doc Al

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    You're asked to find the work done by the tension, so you need the tension. (Since the acceleration is zero, that tension will equal the weight of the object.)
     
  12. Jan 5, 2015 #11
    So its just the weight by the distance seing as the acceleration is zero?

    1250 x 8 = 10000 J ?
     
  13. Jan 5, 2015 #12

    Doc Al

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    Yes.

    1250 is not the weight.
     
  14. Jan 5, 2015 #13
    Sorry i think i get it now

    the weight is the mass times gravity

    1250 x 9.81 =12262.5

    12262.5 x 8 = 98100 J .
     
  15. Jan 5, 2015 #14

    Doc Al

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    Good!
     
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