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

[BrainMan]

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

 

[SammyS]

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.

 

[dauto]

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.

 

[Curious3141]

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.

 

[BrainMan]

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