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A 1200kg car skids with uk = 0.45. If it skids 45.2m, find the thermal energy and the

  1. Oct 25, 2009 #1
    1. The problem statement, all variables and given/known data

    A 1200kg car skids with uk = 0.45. If it skids 45.2m, find the thermal energy and the initial speed.

    m = 1200 kg
    Vi = ?
    Vf = 0 m/s (i think..because it wants to stop)
    uk = 0.45 (coefficient of friction)
    delta d = 45.2m

    2. Relevant equations

    Well for Frictional force we use Ff = uk x Fn (0.45x11760) = 5292N


    3. The attempt at a solution

    So I found the Fn which is 1200 x 9.8 = 11760 N

    Then I found Ff which is 0.45 x 11760 = 5292 N

    Then I found the work done (I don't know why thermal energy is the work can someone explain this please)

    W = F x d = 5292 x 45.2 = 239198.4 J

    But my textbook's answer key has a different answer for the thermal energy, it says it should be 5.3 kJ and 72km/h for the speed. How do we find the speed?
     
  2. jcsd
  3. Oct 26, 2009 #2

    tiny-tim

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    Science Advisor
    Homework Helper

    Hi bigmac! :smile:

    work done = loss in energy (the work-energy equation), and nearly all the energy lost is thermal. :wink:

    So they're different things, but they're equal in value.

    And once you have the thermal energy, you can equate it to ∆(KE) :smile:
    Are you sure it doesn't say 5.3 kN ? :confused:

    Using your figure for W, I do get 72km/h for the speed.
     
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