1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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


    User Avatar
    Homework Helper
    Gold Member

    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

    User Avatar
    Homework Helper
    Gold Member

    Drag and resistance are the same thing!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted