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Power supplied

  1. Jun 9, 2016 #1
    1. The problem statement, all variables and given/known data
    A boat moving at constant speed v through still water experiences a total frictional drag F.
    What is the power developed by the boat?
    a) 1/2Fv b) Fv

    3. The attempt at a solution
    I know the correct answer is b. My confusion is why is a incorrect?
    My reasoning: The boat experiences a total drag of F. Which means at every point the boat experiences a resistance where R < F. Thus to steer at v, we have to apply a force of R. Which will provide the only option 1/2Fv.
     
  2. jcsd
  3. Jun 9, 2016 #2

    Delta²

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    What exactly happens at every point between the boat and the water is a big story of fluid dynamics I doubt we want to discuss for this specific problem. There will be an infinitesimal force ##dF_i## at every point and the sum(more accurately the integral) of all these forces will be ##\vec{F}+\vec{A}=\sum \vec{dF_i}## , where A is the buoyancy force which is cancelled by the weight of the boat. So all that is left is the force F. So we have to supply a force equal and opposite to F to keep the boat move at constant speed v. If you do the math (starting from the definition of the power and also using the definition of work of a force) for a constant force applied at a body that moves with constant velocity v, the power provided by force F is equal to Fv.
     
    Last edited: Jun 10, 2016
  4. Jun 10, 2016 #3

    rude man

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    Drag and resistance are the same thing!
     
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