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Homework Help: Relating speed and engine horsepower of a ship to water resistance

  1. Oct 18, 2012 #1
    1. The problem statement, all variables and given/known data
    The aircraft carrier John F. Kennedy has mass 7.4*10^7kg. When its engines are developing their full power of 280000 hp, the John F. Kennedy travels at its top speed of 35 knots. If 70% of the power output of the engines is applied to pushing the ship through the water, what is the magnitude of the force of water resistance that opposes the carrier's motion at this speed?

    2. Relevant equations
    Kinetic energy: .5mv2
    Work-energy theorem: Work=.5mv22-.5mv12
    Power = Work/Time
    Speed = meters/second
    1 hp= 746 Watts

    3. The attempt at a solution
    Convert horesepower to watts: 280000*746=208880000
    I then tried to muliply this by the speed (forgetting it was speed rather than time) to find the work (forgetting that this wasn't equal to the force of the ship pushing against the water), then said that was the answer due to Newton's third law. Obviously, it didn't work. I am at a loss.
  2. jcsd
  3. Oct 18, 2012 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    Try this:

    Power = Work/time = (Force*distance)/time = Force*velocity
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