# Power and forces

1. Jul 30, 2013

### eroxore

Hello forum.

I am finding it hard to wrap my mind around the concept of power when considering forces. We can derive $\text{P} = \text{F} \cdot v$ but what now does the symbol F really signify?

Is it (1) the net force on an object or (2) can we simply put in any force for F acting on an object? If the net force were to be zero in case (1), then P would be zero but that does not make any sense since driving a car with constant velocity surely requires provision of energy (right?).
In (2), what can we say about a frictional force acting on the object? Does it have som power which can attributed to it?

I would really appreciate if you could help me fathom the relationship between power and forces!

Last edited: Jul 30, 2013
2. Jul 30, 2013

### Staff: Mentor

Power is F*D/t.
Right, so you can calculate power for any force acting in the direction of motion or against it. You could say that the car has a net power of zero acting on it or that it has a power from the engine of X and and a power due to friction of -X.

3. Jul 30, 2013

### eroxore

You are right, sorry about that; all fixed now!

Ok, I then understand that it is not the net force on the object. I understand that it can have power from the engine but the power from air cannot be a property of the car right? The air does work on the car, so it is the rate of the air which is the friction-power, right?

One more thing: If the net force is zero, then you are not changing the speed (energy) of the object and thus it moves in a constant speed. How does power come into play in that case? Moreover, what can be said about power due to the frictional force from the wheels of the car?

4. Jul 31, 2013

### Drakkith

Staff Emeritus
Personally I'd say it's the car doing work on the air, but that's just how I look at it.

You're not changing the speed of the car, but you ARE changing the speed of the air. That's why when you get behind a semi truck you can save a lot of gas. The truck has a small pocket of air behind it that's traveling in the same direction, so you don't do nearly as much work on the air, reducing the amount of fuel it takes to maintain your speed.