# Energy consumed to create force

1. Dec 17, 2008

### Bob Smith

Suppose I have an engine of some kind that, for the purposes of simplification, creates a constant force regardless of conditions.

First, am I correct to assume that the rate of energy fed into this engine will also be constant?

Second, if I know what this constant force is, can I work out how what the energy 'consumption' rate is that is required to maintain it?

2. Dec 17, 2008

Staff Emeritus
No. No work is done until the force is applied through a distance. Whatever energy is applied at this stage is lost - it's not doing work.

3. Dec 18, 2008

### Bob Smith

OK. I understand the energy imparted to an object it might be attached would be zero, but it is the total energy that I am interested in. As far as I'm concerned, the engine could be bolted to the floor, and given a finite energy supply, the energy will, at some point, be depleted. How do I find out how long that would take? What other information am I missing?

4. Dec 18, 2008

### Staff: Mentor

It all depends on the nature of the "engine" that is creating the force. Say I want to create a 100 N force on something. Well, I could use a 100 N rock to produce that force, which would require no energy. There is nothing intrinsic to force alone that requires energy.

5. Dec 18, 2008

### Staff: Mentor

You don't have to use energy to apply the force. Think bolt - the only work done is when you screw it, later it holds on its own.

Edit: Doc was faster. That happens when you have to consult glossary to check "bolt".

6. Dec 18, 2008

### Staff: Mentor

:rofl:

7. Dec 19, 2008

### Bob Smith

OK, I understand those issues now. Let's think of my engine as a fan, so it is the air being expelled from the engine that is creating the force, and here I see is where my work is being done (movement of air, in this example).

I'm still unsure how I get from here to calculate my work done, so I can get the energy required in this system.

Thanks for your help so far, I feel like I'm slowly getting somewhere. :)

8. Dec 19, 2008

### Staff: Mentor

Since the energy (power, really) required to apply a force can be literally anything from zero to infinity, depending on the situation, you need to know exactly what your situation and you haven't given us enough information. If you use a fan to produce a force, you need to know how big the fan is, how much force you want to produce, and if the fan is blowing against a surface or in free air. Then you apply the relevant forms of Bernoulli's equation and a power equation.

9. Dec 19, 2008

### Naty1

Russ Watters has the right idea...practical considerations weigh heavily here....

I have a bit of experience with diesel engines (in boats) as an example....a crude guide is 1GPH of diesel fuel produces roughly a steady 16 to 18 HP....but that varies by compression ratio, by natural versus turbo and by two cycle vs four cycle engine technology, and by mechanical versus electronic fuel injection, for example. In water, three blade props versus four blade props also matters, as does traditional fairing versus computer scanning, as does rotational speed and size of the props...these factors can vary by perhaps 30% to 40% ....