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Work-energy theorem

  1. Sep 17, 2012 #1
    A block (2 kg) is moving with an initial speed of 1 m/s on a horizontal rough table comes to a stop eventually.

    Applying work-energy theorem to the block-table system, we obtain

    Change in the kinetic energy of the block + Change in the kinetic energy of the table = Net work done on the block-table

    (-1 J) + (0) [the table doesn't move] = Net work done on the block + Net work done on the table
    = [Work done by normal force on the block by table + Work done by friction on the block by table + Work done by weight on the block] + [Work done by normal force on the table by block + Work done by friction on the table by block+ Work done by weight on the table]

    Work done by weight on the block is 0 because the angle between the displacement and the weight is 90 degree. Work done by weight on the table is 0 because it doesn't move and therefore has 0 displacement. Work done by the two frictions is also 0 because friction on block by table and friction on table by block are action-reaction pair and thus in general, their resultant is zero and produce zero work. The same goes to the normal force.

    Thus, basically the net work done on the block-table is zero.

    And we get:
    -1 = 0
    What is wrong?
  2. jcsd
  3. Sep 17, 2012 #2
    But you just said the table doesn't move.
  4. Sep 17, 2012 #3
    So, the table should move to fix the -1 = 0?
  5. Sep 17, 2012 #4
    The energy doesn't disappear, it becomes heat.
    The block and table both start radiating in the infrared.
    It is also unlikely the block will come to a complete stop, but will
    "bounce" and thereby consume some more energy.
  6. Sep 17, 2012 #5
    Unless something's holding the table in place. In which case the table and that thing get a slight amount of energy which fixes that. If it's anchored to the Earth, assuming the Earth's perfectly rigid (which it isn't, but assuming,) then it'll have its motion changed slightly to fix that.

    EDIT: Duh. Heat. *Bangs head against wall due to the sheer stupidity of the fact that I overlooked that*
  7. Sep 17, 2012 #6
    If we include heat in the equation, how will it fix the equation?
    Or if the table is free to move, then it will move and gain kinetic energy of 1 J?
  8. Sep 17, 2012 #7
    No, the work is done by the force of friction which acts opposite to the displacement vector.

    There is one joule of heat disippated. This is the work done by the force of friction stopping the motion of the block.
  9. Sep 17, 2012 #8
    I have done some reading to find the answer. This is what I got:

    1. Work-energy theorem applies only to particles.
    2. Objects subject to friction cannot be treated as particles.
    3. Thus it is wrong to apply work-energy theorem to objects subject to friction.
  10. Sep 17, 2012 #9
    From conservation of energy, there is difference in initial and final kinetic energy.
    So the energy must have been converted to other form, conservative or non-conservative.
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