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Homework Help: Power problem

  1. May 16, 2014 #1
    1. The problem statement, all variables and given/known data
    A speedboat requires 130 hp move at a constant speed of 15 m/s. Calculate the resistive force due to the water at that speed.

    2. Relevant equations

    3. The attempt at a solution
    I attempted to use Newtons first law that says in order for a object to be at rest the sum of the forces must be zero. Therefore I thought that the horsepower for the force of friction must be the same as the propulsion. So
    F= 8.6
    The answer is 6.47x10^3 N or 1450 lbs
  2. jcsd
  3. May 16, 2014 #2


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    Staff Emeritus
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    Gold Member

    The problem is with the units.

    Horsepower is not an SI unit.
  4. May 17, 2014 #3
    F = 8.6 what??? What are the units? You forgot to convert the units to some consistent system. hp is a British unit while m/s is International.
  5. May 17, 2014 #4


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    Homework Helper

    There's a standard conversion from hp to kW that every petrolhead will be familiar with. Use that conversion (google it) and work purely in SI units.
  6. May 17, 2014 #5
    OK I see what I did and found out how to solve the problem. Thanks everyone!
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