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Work done problem

  1. Nov 4, 2014 #1
    1. The problem statement, all variables and given/known data
    There is a block of weight mg sitting on a horizontal table with a coefficient of kinetic friction u, at what angle to the horizontal should one direct a driving force to minimise work done in moving the block a horizontal distance of 10m with nonzero velocity and what is the magnitude of that work?
    2. Relevant equations
    W=integral(F.ds)
    F=uR
    3. The attempt at a solution
    Looking at this problem I cannot see why it would not be 90degrees and zero work being done because the direction of motion would be perpendicular to the force so F.ds is 0. But this is a 7 mark question surely that explanation isn't worth 7 marks have I missed something?
    Thank you
     
  2. jcsd
  3. Nov 4, 2014 #2

    Doc Al

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    Staff: Mentor

    That would certainly minimize the work! But would that allow you to move the block as required?
     
  4. Nov 4, 2014 #3
    So would you say 89.999999999.......?
     
  5. Nov 4, 2014 #4

    Doc Al

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    Staff: Mentor

    On second thought, I think you are right. (For all practical purposes: yes, you'd need some slight horizontal component.) I suspect the person creating the problem didn't realize this. (I suspect this is not a textbook problem.)
     
  6. Nov 4, 2014 #5
    It is a college assessed problem bit annoying really
     
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