[PhantomPholly]

Originally Posted by major

Gotta disagree with you here. On the shaft of a motor, even if there is torque, if it is stationary (zero RPM), the mechanical power is zero.

I don't think "power" is a physics term, so whether you are right or wrong on this depends on your subjective meaning of "power."

Torque is a force. At zero rpm, no WORK is done (horsepower is a measure of "work") but there is certainly a force. If the force is 10 ft/lbs and you balance a 10lb weight on the end of a 1 foot pole (of zero mass, for you perfectionists) with the pole horizontal to the ground and the plane of rotation vertical, no work gets done because the force of the motor exactly matches the force of gravity. Lighten the load by a few photons and it starts to move, accelerating away from the earth, and then you may calculate work done over some period of time "X". Make it heavier, and it will begin to descend, but more slowly than if it were in free-fall, and NEGATIVE work would be done (you give up your POTENTIAL energy stored in the form of altitude).

Now there may be electrical power into the motor which produces torque, but the power output is zero. But there could also be a spring which produces that torque. Or a weight on a string around a pulley. Force (or torque) can exist without power.

Regards,

major

So, I think your intent was to say that the instantaneous hp rating is zero, which is true because hp (like "WORK") can only be measured over time.

The most confusing part of all of this is that in the real world our motors are hardly ever making a constant amount of power, unless we are at cruise and have balanced the forces (the instantaneous torque applied to the wheels at each instant is exactly equal to the opposing forces of wind and Rolling Resistance).

In MY personal vocabulary, "the power you are making" would mean the instantaneous measurement of force with the assumption that if you continued that exact force over a finite period of time you could then translate into horsepower. I tend to think in terms of thrust as power, which would be different from what Major is saying (where he is equating power to "work"). Neither is right nor wrong because they are simply English expressions, not physics terms.

[major]

Originally Posted by PhantomPholly

I don't think "power" is a physics term, so whether you are right or wrong on this depends on your subjective meaning of "power."

Hi Phant,

In physics, power is the rate at which work is performed or energy is converted.

from:

http://en.wikipedia.org/wiki/Power_(physics)

That is what we are talking about

major

[IamIan]

Originally Posted by major

Thanks for your input and thoughts. So what is your vote? Torque is irrelevant of relevant?

I don't think it is a universal black and white relevant or irrelevant kind of thing ... I think it is a sliding scale depending on the application... where the value never completely reaches zero.... even if for some applications it can be so close to zero to be virtually the same thing... but depending on the application it becomes more or less relevant... kind of like Aerodynamics on a large cement bridge, might be very minor concern at one application but in another they can become a major dominating concern.

Originally Posted by major

Gotta disagree with you here. On the shaft of a motor, even if there is torque, if it is stationary (zero RPM), the mechanical power is zero. Now there may be electrical power into the motor which produces torque, but the power output is zero. But there could also be a spring which produces that torque. Or a weight on a string around a pulley. Force (or torque) can exist without power.

I agree force even by another name of torque can exist without power...

But I suspect there is more to it than one and only one simple equation.

__ Power goes in - ___ Power in heat , sound, radio waves, etc = __ Power remaining.

Just like F = MA ... M = F/A ... your Mass does not go to zero as the applied force goes to zero... the equation is just an expression of a relationship.

The relationship of the equation that relates power to torque and RPM is only one variation of that equation... there are others which are 100% equally as correct and accurate that do not include a RPM term at all... one such example is electrical power, among others.

The mechanical power out can not exceed the electrical power in... there is a direct relationship between them.

Although I guess that side of it is more academic and deviates a fair bit from the useful common application of the term power... so maybe it is best left for thought experiments just for fun.

[pgrovetom]

The question may not be relevant in a way. Since Torque x RPM = k x Power, the torque curve and power curve can be derived from one another. So both are relevant but not more relevant than the other.

If you give a Torque versus RPM curve, the power versus RPM can simply be calculated and visa versa. The shape and area under the "curves" is important and relevant.

The confusion probably comes from the RPM ( and some constant) is used in the translation. If one just talks about Torque without regard to an RPM, then - how power and torque relate is confused.