What is the power of a car rolling at 50 km/h? I am trying to understand what would be the power difference in watt for making rolling a car of 1000 kg and a bycycle of 80 kg (human + bycycle weight) on a plate road.
My car as around 100 horsepower. But it does not mean that it needs 100 horsespower for rolling at 50 km/h on a plate road.
Does anyone can help me, please?
 A: You are confusing power with energy.
The power exerted by a rolling vehicle at a given speed is equal to the summation of all drag forces times the velocity of the vehicle; this is the rate of work required to push it down the road at constant velocity.  Even at 60MPH, this drag horsepower is a just a fraction of the full power available from the engine when it is running "wide open", screaming along at full throttle and maximum RPM's.
For example, the drag horsepower of my pontiac vibe cruising at 65 MPH on level ground with no headwind is about 18HP. Its maximum available power is about 132HP. Note that the drag horsepower scales as the cube of the velocity, which means that to double the speed of a vehicle requires 8 times the horsepower.
On the other hand, the kinetic energy of a moving vehicle is equal to 1/2 x (mass) x (velocity)^2. This is a measure of how much damage the vehicle can do if it collides with another object.
A: Ok its very simple.
Energy = 0.5 m V^2  so E=0.5 x 1000kg x 13 X13 = 84500 Joules is needed (forget the wind drag on the car & friction)
Now if you want to finish that energy in 30 minutes then time =1800 seconds
So Power = E/t  = joules / second
So Power needed for that 1/2 hour is:   P= 84500J/1800s = 46.9 Watts is what you need power wise.
Remember to add friction and drag to get a realistic answer.
Also if you want to accelerate 0-50km/h in 2 seconds then you need much more power
Power for 0-50 = E/t = 84500J/2seconds = 42250 Watt will now be needed by the engine. This is like a thousand times bigger engine and thats why your Mustang is 200 horses. So we dont need big engines to cruise, but we need them mostly to accelerate.
Giel
A: If something is said to be operating at 100 horsepower, then that is the power output. If it was more complicated than this for the problem you are solving then more information would, or at least should, be given.
We know instantaneous power $P$ delivered by a force $\mathbf F$ is given by $P=\mathbf F\cdot\mathbf v$, where $\mathbf v$ is the instantaneous velocity of the object the force is acting on. If we want to simply to one-dimensional motion, we have $P=Fv$. At a constant power, this just means that $F$ and $v$ are inversely proportional: $F=P/v$. As the object speeds up, the applied force decreases.
