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Work and Energy of snow

  1. Feb 16, 2015 #1
    1. The problem statement, all variables and given/known data
    A sled is being pulled across a horizontal patch of snow. Friction is negligible. The pulling force points in the same direction as the sled's displacement, which is along the +x axis. As a result, the kinetic energy of the sled increases by 38% . By what percentage would the sled's kinetic energy have increased if this force had pointed 62o above the +x axis?

    2. Relevant equations
    The Work-Energy Theorem

    3. The attempt at a solution
    image.jpg

    So I get, that the kinetic energy is reduced by 35% .

    Correct answer: the kinetic energy increases by 18% .
     
  2. jcsd
  3. Feb 16, 2015 #2

    Bystander

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    If the problem statement is complete and correctly transcribed in your original post, without checking your math, you are correct. It would be best that you double-check the problem statement.
     
  4. Feb 16, 2015 #3
    The problem statement is correct 100%. Maybe I understood the problem incorrectly?
     
  5. Feb 16, 2015 #4

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    As written, what you've done looks correct --- the x-component of the force is 100 % for the first case, and is reduced for the second. Looks like a really lousy problem statement, and an even worse job of proof-reading/checking matches of answers with problems. Let us know whether anyone fesses up, or how they rationalize the answer that's claimed.
     
  6. Feb 16, 2015 #5

    BvU

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    I think it is understanding indeed. I fail to understand what W0 represents, for example.
    You are given that Ekin + F ##\cdot## s = 1.38 Ekin

    Now try to grasp what you are being asked. The 17.8% book answer is correct.
     
  7. Feb 16, 2015 #6
    Wo represents work with which other works (W1 and W2) are compared.

    I understand your method, but I don't understand why my method didn't work.
     
  8. Feb 16, 2015 #7

    BvU

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    Still don't understand. If you write W1 = F ##\cdot## s , then I don't understand why you consider that to be 138 % of W0 instead of 38%
     
  9. Feb 16, 2015 #8
    As the problem says, the kinetic energy of Wo increases by 38% .

    If I say, that the very first energy, with which we will compare other energies, is equal to 100% , then the second energy W1, which is along +x axis, must be equal to 138% (increases by 38%) .
     
  10. Feb 16, 2015 #9

    BvU

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    Fine with me. But then you must write W1 = W0 + F ##\cdot## s.
    There is no friction, the original kinetic energy doesn't go away.

    And energies are not along any axis. Energies are numbers. But I understand what you mean.
     
  11. Feb 16, 2015 #10

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    BvU has you pointed in the right direction --- stay with him, and forget everything I said --- I misread the problem horribly.
     
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