# Power problem

1. May 16, 2014

### BrainMan

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
P=F(V)

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

2. May 16, 2014

### SammyS

Staff Emeritus
The problem is with the units.

Horsepower is not an SI unit.

3. May 17, 2014

### 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.

4. May 17, 2014

### Curious3141

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.

5. May 17, 2014

### BrainMan

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