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Homework Help: Work done on a block.

  1. May 14, 2013 #1
    A block of 0.5kg is pulled up an inclined plane by a tensile force at a constant velocity of 2m/s. Said block is pulled 15m up the incline. Sin(theta) = 1/3. The coefficient of kinetic direction is 0.08. Calculate the work done on the bock. Also, calculate the power applied to the block.

    Relevant equations:
    F(friction) = coefficient of kinetic friction x Reaction force.

    Attempt at solution:
    I'm not sure if I'm looking at this correctly but it seems to me that the net force will be 0. If it's pulled up at constant velocity then the net force is 0 and thus the net power and work is also 0. Is this correct?
  2. jcsd
  3. May 14, 2013 #2
    Think about this. If your car is traveling at a constant 20 mph, is the engine doing any work?
  4. May 14, 2013 #3
    Well, yes. It would be. I understand that the tensile force acting on the block will do work on the block. This tension will be equal to the sun of the other two opposing forces. Work will also be done against the other two forces. So, summing up to get the net work on the object wouldn't it sum up to be zero? You'd have the tensile force times the distance - the work done against the other two forces times the distance. Wouldn't this evaluate to 0?
  5. May 14, 2013 #4
    Some of the work by the tensil force might go into increasing the potential energy, yes? Some of it might go into heat of friction? yes.
  6. May 14, 2013 #5
    Ok. Am I just looking for the work done by the tensile force so? I'm not sure I follow what you're getting at.
  7. May 14, 2013 #6
    Figure out what the tensile force is, then

    W = FXD, yes?
  8. May 14, 2013 #7
    Yup. That's great. Just misinterpreted the question so. Thanks for that.
  9. May 14, 2013 #8


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    I guess the problem is asking for net work and power, not work or power done by the tensile force against friction and gravity. So yes, net work and net power done by all forces evaluate to 0. You might want to consider the work energy theorem......net work done on an object by all forces acting on that object is equal to its change in kinetic energy. Any KE change?

    Edit: Question is subject to misinterpretation.
  10. May 14, 2013 #9
    Good point Phantom. The question was not the work done by the tensile force but the work done ON THE BLOCK. Wouldn't a change in potential energy be considered work on the block?
  11. May 14, 2013 #10
    Well, there'd be no change in kinetic energy. It's potential would obviously change though, yeah.
  12. May 14, 2013 #11
    The heat generated by friction would heat up the surface and the block, so some (not sure how much) of the friction work or heat would apply to the block, and some to the surface??
  13. May 14, 2013 #12
    And this would account for the change in potential, yes?
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