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Homework Help: Work in walking?

  1. Nov 10, 2007 #1
    1. The problem statement, all variables and given/known data
    my teacher says that there is no work done on carrying a bag walking horizontally but there is work done in walking upstairs. why?

    2. Relevant equations
    work is done with motion in the direction of the force/ with displacement. mathematically; force x distance

    3. The attempt at a solution
    when the teacher first asked i answered there is work because it's just like pushing the bag but the difference is that the bag is elevated the other thing that lead me to that was that the force from the legs' direction is forwards and the bag moves forward too, i thought my answer made sense because the displacement was in the direction of the force. later i found out that i was wrong because they said force to be considered was the force of the arm and since the bag wasn't displaced upwards (the direction of the force of the arms); no work. so, i was wrong. i didn't quite accept that because when you illustrate it (| is the person and o is the bag) |o.........|o it obviously shows that the bag was displaced from its original position and moved to the direction of the force of the legs. then someone said (i think it was my teacher or my classmate) that the point of origin was your body and since the bag didn't move away from the body..no displacement no work. that to me finally made sense. then the teacher asked if there was work done walking upstairs; i said none because the bag wasn't displaced away from the body. i was also wrong then. the reason now was that the bag moved up and the direction of the force was also up. then i got confused again because walking up, the bag doesn't move away from your body and how come walking horizontally didn't qualify as work when the bag still moved in the direction of the force!!
  2. jcsd
  3. Nov 10, 2007 #2


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    When you are walking horizontally with the bag, the gravitational force is acting on both of you. And the height of the bag and center of mass of your body remains the same. So ther is no change in the PE, and hence no work is done. When you climb vertically with bag height of both of you increases, PE increases and hence work is said to be done.
  4. Nov 10, 2007 #3


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    double post.
  5. Nov 10, 2007 #4
    just waking...is your potential energy changed? and how else can potential energy be changed other than raising?
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