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Energy Lost due to friction

  1. Jan 9, 2015 #1
    1. The problem statement, all variables and given/known data
    A team of dogs drags a 114 kg sled 1.92 km over a horizontal surface at a constant speed. The coefficient of friction between the sled and the snow is 0.124. The acceleration of gravity is 9.8 m/s 2 . Find the work done by the dogs.

    Answer in units of kJ.

    Find the energy lost due to friction. Answer in units of kJ.

    2. Relevant equations
    I don't know, I think that might be part of my problem

    3. The attempt at a solution

    For the first one, I did 114 x 9.8 x 1.92 x 0.124 to get 265.982976, which was correct

    but for the second part, I tried to do 114 x 9.8 x 1.92 to get 2145.024, which would be the energy with no friction, right? Then I subtracted the energy with friction from the energy without friction to get 1879.041, but that was wrong. What am I doing wrong? Wouldn't it make sense that you could find energy lost like this? Is there a formula?
     
  2. jcsd
  3. Jan 9, 2015 #2

    haruspex

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    You calculated the work done by the dogs. Where has that work gone? PE? KE? Anywhere else?
     
  4. Jan 9, 2015 #3
    Wouldn't it all have gone to KE?
     
  5. Jan 9, 2015 #4
    Sorry if I'm completely missing your point, I missed the class where they talked about this so I'm a bit lost
     
  6. Jan 9, 2015 #5

    haruspex

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    How do you assess change in KE?
     
  7. Jan 9, 2015 #6
    I know that the kinetic energy formula is 1/2mv^2, so to find change in KE I would just use that formula to get the KE and subtract final KE from initial KE
     
  8. Jan 9, 2015 #7

    haruspex

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    Right. Is the speed changing?
     
  9. Jan 9, 2015 #8
    no, the speed is constant
     
  10. Jan 9, 2015 #9
    that wouldn't make the answer 0, would it?
     
  11. Jan 9, 2015 #10
    it isn't, can I have another clue, please?
     
  12. Jan 9, 2015 #11

    Nathanael

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    What is the change in energy of the sled?
     
  13. Jan 9, 2015 #12
    Wouldn't it also be 0? Becasue the sled is attached to the dogs
     
  14. Jan 9, 2015 #13

    Nathanael

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    So if the sled didn't gain any energy, what happened to the energy (from the work done by the dogs) from part one?
     
  15. Jan 9, 2015 #14
    It became friction energy?
     
  16. Jan 9, 2015 #15
    does that even make sense?
     
  17. Jan 9, 2015 #16

    Nathanael

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    Right.
    Well... more correctly, you should say something like, "it was lost through friction." But you have the right idea.

    It wouldn't be called "friction energy", it would be in the form of "thermal energy" (because the snow would be a bit warmer).

    Haha, not exactly, but you have the right idea.
     
  18. Jan 9, 2015 #17
    Okay I think I'm starting to get it, all of the energy has to go somewhere, so in this case it all became thermal energy
     
  19. Jan 9, 2015 #18
    YES! Thank you so much!
     
  20. Jan 3, 2016 #19
    Wait, but that still doesn't answer the amount of energy lost! What's the formula?
     
  21. Jan 3, 2016 #20
    you don't need a formula for this one
     
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