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B Question about Work

  1. Jan 13, 2017 #1
    Okay...I have an example of work calculation in my book in which a girl has carried a bag upstairs( i.e. to some height)...The force the girl has applied is taken equal to the weight of bag( W )...According to my understanding, The girl will apply a force greater than W when she lifts it up from floor, will apply a force equal to W in transit, and will apply a force less than W when putting it down..(Correct me if wrong)...I have a question here that why we are taking the force equal to W when calculating work when that force is not actually causing displacement?????
     
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  3. Jan 13, 2017 #2

    fresh_42

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    You don't have to take it as equal but both quantities are proportional, so as longs as you don't want to have some exact figures according to a certain system of units, one could as well see the proportion factor to be one. Why do you think there is no displacement? She carried the bag upstairs, didn't she? Do you know the connection between force and work in this context?
     
  4. Jan 13, 2017 #3
    Work is done when a force displaces a body(in its direction)...The girl applied a force (>W) when picking up the bag which give it some vertical velocity...However during her transition from down to upstairs, she does'not cause any net force on the bag...(and this is time time when she is applying W force)...In other words , the W force did not cause displacement, the >W force did....
     
  5. Jan 13, 2017 #4

    fresh_42

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    She increased the potential energy of herself and the bag, because she extended the distance between the two masses earth and bag. She applied a force opposite to the gravitational force of the earth.
     
  6. Jan 13, 2017 #5

    A.T.

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    Because "causation" is irrelevant to the definition of work.
     
  7. Jan 13, 2017 #6

    sophiecentaur

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    Yes. It's Work Done On and not Work Done By. However the bag gets up there, the work is the same.
     
  8. Jan 13, 2017 #7

    A.T.

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    No idea what that means. My point was that "the cause of motion" is irrelevant to computing work. In fact "the cause of motion" is ambiguous, because motion (and work) depend on the choice of reference frame, while the force (as the supposed "cause") doesn't.
     
  9. Jan 13, 2017 #8
    The OP seems not to understand that the relevant displacement is the elevation change from downstairs to upstairs. The lifting of the bag from the floor to the carrying position is also a displacement normally ignored in such examples. The final act of letting the bag back down to the floor (the upstairs floor) cancels that out anyway. The only things that matter (to the "net" work) are the initial and final elevations.
     
  10. Jan 13, 2017 #9
    Work depends on the frame of reference. From the frame of reference of the earth, there is an upward force exerted on the bag, and it is displacing upwards, so work is done (as reckoned from this frame of reference). From the frame of reference of the girl, there is an upward force exerted on the bag, but it is not displacing upwards, so no work is done (as reckoned from this frame of reference).
     
  11. Jan 13, 2017 #10
    Which did not cause any kind of displacement in the body...Which just stopped ''retardation'' of bag...(i dont know if stopping retardation is considered as causing displacement)...If that force did not cause displacement, then it should not do work...??
     
  12. Jan 13, 2017 #11
    Work is when a force displaces a body...In the considered case, the force does not displace a body...(unless stopping retardation is considered a displacement)
     
  13. Jan 13, 2017 #12
    As I said, as reckoned from the girl's frame of reference, no work is done. When one is talking about the amount of work done, one must specify what frame of reference is being used to reckon the work.
     
  14. Jan 13, 2017 #13

    jbriggs444

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    Work is done when a force is applied to a body which is displaced. Regardless of what other things might be deemed to cause the displacement.
     
  15. Jan 13, 2017 #14
    Kaneki, why are you saying there's no displacement? The bag was moved to a higher elevation. That's a displacement.
    Now, if the girl was riding an escalator up while carrying the bag, then she didn't do the work of raising the elevation of the bag. But if she climbed the stairs while carrying the bag, she did the work. She had to counter the weight of the bag by pushing down on each step with her feet.
     
  16. Jan 13, 2017 #15
    That is the work done on the bag, as reckoned from the laboratory frame of reference. As reckoned from the girl's frame of reference, there was no displacement and no work done on the bag (by the girl). There is no inconsistency here.
     
  17. Jan 13, 2017 #16

    jbriggs444

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    As I understand @Kaneki123's concern, it is not the reference frame which is the problem. It is the question of causation -- the [mistaken] idea that a force only does work when it causes motion.

    The lack of a displacement is noted not because @Kaneki123 is considering the girl to be at rest. Rather, it is because the bulk of the displacement takes place during a steady state when the bag ascends the stairs at a steady rate under a net force of zero. During this phase, the support from the girl and the down-force from gravity are in balance. @Kaneki123 sees this as a condition where the movement which she agrees does exist does not count toward work done because the force applied by the girl during this interval does not cause the motion.

    However, as @A.T. has pointed out multiple times, the notion of causation is simply irrelevant.
     
  18. Jan 13, 2017 #17
    @Kaneki123 Is that actually what you are saying?
     
  19. Jan 13, 2017 #18

    A.T.

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  20. Jan 14, 2017 #19
    Okay I understand now...One last thing, if work is done when displacement is in same direction as force, so where does the concept of ''negative work'' comes from???
     
  21. Jan 14, 2017 #20

    jbriggs444

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    When displacement is in the direction opposite to the force being considered.

    For example, the force of a man's hands on a thrown ball that he is catching.
     
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