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B Applied force

  1. Sep 27, 2016 #1
    Okay, I just want to confirm...When we take an object to some height, let's say a bag taken upstairs, Do we have to apply the upward force equal to the weight of object or more than the weight of the object...???
    I know that its more than weight..But just want to confirm...
     
  2. jcsd
  3. Sep 27, 2016 #2

    A.T.

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    What does Newtons 2nd Law say?
     
  4. Sep 27, 2016 #3
    An applied net force causes a body to accelerate...There should only be a net force of the object if the upward force is greater than weight...Right???!!!
     
  5. Sep 27, 2016 #4
    Do you talk about the forces when de bag falls? or the forces when you take the object upstairs?
    because in the first situation your netto will be downward because it is accelerating(until air restsistance is equal to the mass*g)
    in the second situation, it depends is you accelerate or not, if not your force must be equal to the mass of the bag times 9,81.
     
  6. Sep 28, 2016 #5

    jbriggs444

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    Yes, that is right. So we can safely conclude that if the body starts at rest on the basement floor and transitions to a state where it is moving up the stairs, an average upward force exceeding gravity is required during that transition.

    That's only one of three pieces of the problem taken care of. What about the period while the object moves up the stairs at constant speed? And what about the transition at the top of the stairs when the object is placed back on the floor?
     
  7. Sep 30, 2016 #6
    Okay, the problem was like this
    We had to calculate work done while taking a bag upstairs...The amount of force was taken to be equal to weight...From what I understand the force needed to move the bag upstairs SHOULD be greater than its weight...right???
     
  8. Sep 30, 2016 #7
    Okay,I think i was approaching the problem the wrong way..Can you please elaborate as to what is the relation between the application of force and constant speed???
     
  9. Sep 30, 2016 #8
    Can you please elaborate???
     
  10. Sep 30, 2016 #9

    A.T.

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    For that you either have to integrate or take the average force.

    What is the average force, if the bag ends up with the same vertical velocity that is started with?
     
  11. Sep 30, 2016 #10

    jbriggs444

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    Newton's second law is good. If velocity is constant then acceleration is... what?

    The easy way to calculate work done is by energy conservation. Then one need not determine the required force pattern.
     
  12. Sep 30, 2016 #11
    zer
    It should be zero...
     
  13. Sep 30, 2016 #12
    okay, so by my understanding...you mean to say that the only acceleration will be of picking up of bag and the first step taken...then the velocity will remain constant...right???
     
  14. Sep 30, 2016 #13

    jbriggs444

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    Right. At least up until the point where the last step is taken and the bag is set back down. That also involves an acceleration.
     
  15. Sep 30, 2016 #14
    If we are to see it like this...The moment when the person lifts the bag up from floor, he has to apply an upward force greater than weight of bag..There is the acceleration...The first step he take, he has to apply a force greater than his and bag's weight combined...There is an acceleration...Now lets assume that he somehow ''floats'' up to the last step with constant velocity while holding the bag...So is it true that he will not have to apply a force greater than the weight of bag (while being in constant velocity) but equal to weight, As long as the bag has same velocity as him???
     
  16. Sep 30, 2016 #15

    jbriggs444

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    For every step you start where you are applying an upward force that is momentarily greater than the bag's weight there is a step you stop where you are applying a force that is momentarily less.

    No matter what you do, if the bag ends up at rest, the average force will turn out to have been equal to the weight of the bag.
     
    Last edited: Sep 30, 2016
  17. Sep 30, 2016 #16

    A.T.

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    If the average net force is zero, what is the average applied force?
     
  18. Oct 1, 2016 #17
    You picking up bag taking it upstairs, is'nt that acceleration???
     
  19. Oct 1, 2016 #18
    How can you be at constant velocity while taking the bag upstairs???
     
  20. Oct 1, 2016 #19

    jbriggs444

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    Why would you think that you could not be?
     
  21. Oct 1, 2016 #20
    For every step you take, you apply an upward force, as a result of which you accelerate upwards...So how can you be at constant velocity???
     
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