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Gravitational and kinetic energy problem

  1. Jan 20, 2007 #1

    lzh

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    1. The problem statement, all variables and given/known data
    Given: 1 hp = 746 W and 1 m/s =2.237 mi/h.
    When an automobile moves with constant
    velocity, the power developed in a certain en-
    gine is 85.4 hp.
    What total frictional force acts on the car
    at v = 72.8 mi/h? Answer in units of N.


    2. Relevant equations
    Kinetic Energy=.5mv^2
    Disspated energy=F*displacement <-the f would be the frictional force.
    W=change in energy(J)/change in time(s)
    3. The attempt at a solution
    i first converted 85.4 hp to W: 63708.4W
    then the v to m/s: 32.54m/s

    i would like to find the amount of joules in 63708.4W, but I dont know the time. So I probably need to find a way to cancel it.

    also, I don't know the mass of the car...nor the displacement. I'm not sure of what to do with so many unknowns.
    Can someone point my problems out?
    Ty in advance!
     
  2. jcsd
  3. Jan 20, 2007 #2

    arildno

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    Why should you want joules for??

    Instead, how are the quantities power, force and velocity related?
     
  4. Jan 20, 2007 #3

    lzh

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    power=Force*displacement/time
    after some simplification
    I'm starting to see something though
    edit:i forgot to type /t
     
    Last edited: Jan 20, 2007
  5. Jan 20, 2007 #4

    arildno

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    You have constant velocity. What is therefore displacement/time equal to?
     
    Last edited: Jan 20, 2007
  6. Jan 20, 2007 #5

    lzh

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    velocity!
    i see, ty
     
  7. Jan 20, 2007 #6

    lzh

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    so that means Force*v=63708W
    substituting v i get:
    F=1957.84N
    but thats not the frictional force, I'm going to solve for that next
     
  8. Jan 20, 2007 #7

    arildno

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    So, what you have found is the force the engine provides on the system, agreed?

    Now, how much acceleration does the car experience if it moves with constant velocity?
     
  9. Jan 20, 2007 #8

    lzh

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    yeah thats the force of the engine.
    car at constand velocity has 0 accelaeration
    so at constant velocity, the force of friction must be equal to force of engine?
    so F=1957.84N is correct?
     
    Last edited: Jan 20, 2007
  10. Jan 20, 2007 #9

    arildno

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    Correct!:smile:
     
  11. Jan 20, 2007 #10

    lzh

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    ty!
    it was correct, according to my utexas's hw service
     
  12. Jan 20, 2007 #11

    arildno

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    How fortunate for the hw service! :smile:
     
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