# Engine Performance(Finding Power)

1. Sep 26, 2004

### austin zitro

Ok, here is the problem:
A car manufactuerer claims that their car can accelerate from rest to 101 km/hr in 5.6 seconds. The car's mass is 840 kg. Assuming that this performance is achieved at constant power, determine the power developed by the car's engine.
Part B) What is the car's speed after 2.8 s?

Ok, so first off i changed the 101km/hr to m/s. So it should be 28.1m/s. Then I used the equation: V=Vo + at to find acceleration and so plugging in 28.1m/s=0 + a(5.6s), a= 5.01m/s^2. So since power = F*v, I plugged in P = 840kg*5.01m/s^2*28.1 = 118256.04 W. This is not right. Since the power is constant, I'm thinking that the acceleration is probably not. So how would I incorporate that into my equation?? Once I get that, I can prolly get part B. Thank's for any help at all!

2. Sep 26, 2004

### Pyrrhus

Check those units are those the Power units?? also think about constant power what does that mean?

3. Sep 26, 2004

### austin zitro

I ended up getting it. I used the equation P=W/t, where work = change in kinetic energy. Thanks for the help.

4. Sep 27, 2004

### Tide

You know that
$$F = m \frac {dv}{dt}$$
What can you conclude if Fv is constant?