Relating speed and engine horsepower of a ship to water resistance

  • Thread starter wowser
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Homework Statement


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?



Homework Equations


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

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.
 

Answers and Replies

  • #2
cepheid
Staff Emeritus
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Try this:

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

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