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Help understanding dynamic equilibrium

  1. Dec 26, 2012 #1
    1. The problem statement, all variables and given/known data

    if the equilibrium rule states ƩF=0N, how can something move or be moving at a constant speed ? eg I push on a box with 50N and friction provides 50N back, resulting in ƩF=0N. So I am not providing more N than the box is resisting with. I thought something could only be moved if the moving force was greater than the resisting force ? otherwise wouldn't it be a stand still ?
    2. Relevant equations



    3. The attempt at a solution

    ****update*****

    just had thought...I'd only need more force than the friction provided to get the box moving, but once moving and if at a constant speed then its motion is unchanged, thus the zero net force ?? And this then becomes the first law as now it is moving it will remain moving until an unbalanced force acts upon it ??
     
    Last edited: Dec 26, 2012
  2. jcsd
  3. Dec 26, 2012 #2

    gneill

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    Staff: Mentor

    Newton's first law. If the ƩF=0N, no change in motion occurs.
     
  4. Dec 26, 2012 #3
    What if the box is sliding on the table by itself and there's no friction?
     
  5. Dec 26, 2012 #4

    gneill

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    Staff: Mentor

    If there's no friction, what's the net force acting?
     
  6. Dec 26, 2012 #5
    That was directed at the OP, in hopes of him being able to figure out the answer to his question.
     
  7. Dec 26, 2012 #6

    gneill

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    Ah. Sorry for pouncing then :blushing:
     
  8. Dec 27, 2012 #7
    hmmm I dont quite get what would make the box slide ? If ƩF=0 and there's no friction my guess would be there is no force...but then I don't get how the box would be sliding with no force on it
     
  9. Dec 27, 2012 #8

    gneill

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    Staff: Mentor

    What if it were already sliding?

    Motion does not require a force. Only a change in motion requires a force.

    This is an essential difference between Aristotelian physics and Newtonian physics :wink:
     
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