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Homework Help: Work, Force, Energy and power question about moving car.

  1. Apr 8, 2015 #1
    1. The problem statement, all variables and given/known data
    A car has a power output of 45kW and travels at a constant speed of 30m/s. What force is the engine applying on the car?

    2. Relevant equations

    3. The attempt at a solution
    I really have no clue. Since there is no change in energy (ΔE) in terms of kinetic or gravitational potential energy, I don't know what to do.

    Any help is much appreciated.
  2. jcsd
  3. Apr 8, 2015 #2
    Does F*v (Force*Velocity) sound familiar to you ?
    [Edit: In case not, power is change of energy over change in time, so P = ΔW/Δt = Δ(F*x)/Δt = F*Δx/Δt = F*v]
    Last edited: Apr 8, 2015
  4. Apr 8, 2015 #3
    Yes, but usually only in the context of force of friction where

    P=Ff*d/t and as such P=Ff*v.

    Am I missing something about this question?
  5. Apr 8, 2015 #4
    You're right, but not only for friction,
    In general, P = dW/dt so you can work it out here :)
  6. Apr 8, 2015 #5
    Hmm.. so with that I get,

    45kW=ΔF*d/Δt---since ΔF*d or ΔKE is 0, does that mean the engine is not applying any force on the car?
  7. Apr 8, 2015 #6
    If the engine is applying some force on the car, it would be accelerating and but it's moving at a constant speed (and have a mass) so it has kenetic energy in fact, but it's not the purpose of the exercice I guess, it would be better to consider, in this case that P = F*v and take a force out of that :)
  8. Apr 8, 2015 #7
    Aha! I understand. Thanks for the help mate :)
  9. Apr 8, 2015 #8
    Welcome ^^ :)
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