Calculate Resistive Force of Water on Speedboat | 130 hp Required for 15 m/s

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Homework Help Overview

The discussion revolves around calculating the resistive force exerted by water on a speedboat that requires 130 horsepower to maintain a constant speed of 15 m/s. The problem involves understanding the relationship between power, force, and velocity.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants discuss the application of Newton's first law and the relationship between horsepower and resistive forces. There are questions regarding unit consistency, particularly the conversion between horsepower and SI units.

Discussion Status

Some participants have identified issues with unit conversions and the need to use consistent measurement systems. Guidance has been offered regarding standard conversions from horsepower to kilowatts, but there is no explicit consensus on the final approach to the problem.

Contextual Notes

There is an acknowledgment that horsepower is not an SI unit, which raises questions about the appropriateness of the units used in the calculations. Participants are exploring the implications of this on their reasoning and calculations.

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Homework Statement


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.


Homework Equations


P=F(V)


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
130=F(15)
F= 8.6
The answer is 6.47x10^3 N or 1450 lbs
 
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BrainMan said:

Homework Statement


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.


Homework Equations


P=F(V)


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
130=F(15)
F= 8.6
The answer is 6.47x10^3 N or 1450 lbs
The problem is with the units.

Horsepower is not an SI unit.
 
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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.
 
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BrainMan said:

Homework Statement


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.


Homework Equations


P=F(V)


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
130=F(15)
F= 8.6
The answer is 6.47x10^3 N or 1450 lbs

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.
 
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OK I see what I did and found out how to solve the problem. Thanks everyone!
 

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