# Power needed to keep car moving

1. Nov 22, 2007

### VanKwisH

1. The problem statement, all variables and given/known data
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 travelling at a constant 80km/h?

2. Relevant equations
P = W / t

3. 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?

2. Nov 22, 2007

### 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

3. Nov 22, 2007

### 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

4. Nov 22, 2007

### 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

5. Nov 22, 2007

### VanKwisH

wha ??? so the answer is????