Power needed to keep car moving

[VanKwisH]

Homework Statement

A 1200 kg car slows down from 90km/h to 70 km/h in about 5.0s on the level when it is in neutral. Approximately what power ( in watt's and hp ) is needed to keep the car traveling at a constant 80km/h?

Homework Equations

P = W / t

The Attempt at a Solution

I really have no idea ... and
how much does 1hp = in watts and in joules ?
550 ft lbs/s = in joules and watts?

 

[rock.freak667]

$$P=\frac{dW}{dt}=\frac{d}{dt}(Fs)=F\frac{ds}{dt}=Fv$$if you have the change in velocity and the time it acts for...you can find acceleration and F=ma...so you can find the power

 

[VanKwisH]

what does each of ur variables mean?? cause i think i use a different variables than u do and i find it confusing to understand

 

[rock.freak667]

Oh well from the definition of power, the rate at which work,W, is done =$\frac{dW}{dt}$ since work done= Force(F)*distance(s) i.e.$\frac{dW}{dt}=\frac{d}{dt}(Fs)$ since force is constant you can then re-write it like $\frac{d}{dt}(Fs)=F\frac{ds}{dt}$ and well $v=\frac{ds}{dt}$

In short: F=Force of car
P=power
v=velocity of car

 

[VanKwisH]

wha ? so the answer is?