Work done problem

  • Thread starter sorax123
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  • #1
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Homework Statement


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?

Homework Equations


W=integral(F.ds)
F=uR

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
 

Answers and Replies

  • #2
Doc Al
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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.
That would certainly minimize the work! But would that allow you to move the block as required?
 
  • #3
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That would certainly minimize the work! But would that allow you to move the block as required?
So would you say 89.999999999.......?
 
  • #4
Doc Al
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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.)
 
  • #5
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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.)
It is a college assessed problem bit annoying really
 

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