Random Babbling Thread CXVIII

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Well, to be more precise, it's ft.-lbs. per second. Foot-pounds is a measure of work; power is the rate at which that work is done.

Not really. Mass is mass. Depending on the context, pounds can either be a force (pound (force); weight would be the most common example) or mass (pound (mass)). But in the context of power, the lb. is always a reference to force.

No. Watts is power. Nm is torque. Two different things (although power is derived from torque by formula).

Edit: BTA beat me to it (kinda)

 

Good point, although I was thinking in terms of engines (which seems to be what henkie was originally talking about), so torque seemed more applicable. One thing’s for sure, though – Nm isn’t a measurement of power, which was my main point.

Now all that’s needed is for someone to say that, to maximize a car’s acceleration, it should be kept at the engine’s torque peak. Then you and I can gleefully shoot him (or her) down!

 

I really appreciate the nice things people do .

 

BTA and Splunge, you are both quite right, of course, I missed the per second in the lbf.ft/s. And of course W = J/s = Nm/s = kgm^2/s^3.

Ah, but I have no context when considering imperial units, so what is obvious to you, is confusing to me because I don't know that lbs in the context of power means force pounds.

It's also not necessarily logical, because work might also be done by kinetic energy, which is typically defined as mass times velocity squared. No obvious link to pound force there.

 

I really appreciate pancakes.

 

Actually, in the context of engines, it's typically per minute, not per second (that was my mistake).

My point was that, when talking about power, the reference is always to force, not mass (which I though was what you were suggesting when you talked about pound mass)

Not quite. Work is done by force. Kinetic energy is the result of work having been done (the change in KE is the work done, and that rate of change in KE, which is also the rate of work done, is power.) EDIT: although work done on one object could be the result of the KE transferred from another object.
And KE is 1/2 of mass times velocity squared.

 

Of course, it makes perfect sense in SI units, but not in imperial units, not to me.

The power reference was also just something I noticed on the wikipedia page. The real trauma I have with imperial units comes from the definition of a specific kind of speed that takes heat (pick your imperial unit), some gas constant in ft.lb/lb.R and some more stuff, where it's not readily apparent what unit is refered to. And no background knowledge of any context. Although I suppose THE gas constant is defined in kJ/kg/K, which would indicate that the gas constant refered to in that paper would be ft.lbf/lbm.R. Unless it's another definition, of course.

Anyway...

Power is always defined per second to my knowledge, though engine revolutions are normally per minute, yes.

That was what I was talking about, yes. And yes, if you stop to think about it, I suppose it makes sense. Except it's still confusing to use the same abbreviation for two different things.

Yes, half the mass etc. I should be more precise.

What I was trying to clumsily and hastily say is that power can be derived from many different things that do not necessarily imply force. For an engine you could say work is done by force as defined and measured by the output torque. But the output torque comes from the pressure and a change in volume. Which in turn is produced in fuel combusting in an air mixture.

P = M/N = W/N = p dV = Q dt = T dS dt
(which doesn't make an awful lot of sense in the limited make up available on the forum, but essentially:
Power = Torque per revolution = Work per stroke time number of strokes per second = pressure integrated over the volume change in a cylinder (times nr of cylinders and nr of strokes per second etc) = integral of heat released by combustion over time = integrated temperature as function of entropy over time.)

By the end the relation to force is not directly apparent to me, nor do I have a context for the imperial units for the entropy.

Like I said above, the power case is fairly simple to deduce, but some others are not. And that's where mars landers start crashing into mars instead of landing on it.*

* Well actually that was a case of NASA working in imperial units and ESA working in SI units. Good job, guys.

 

So, ultimately, we all agree that the imperial system sucks.


I think I'll be making another online RP group soon. My IRL group is getting harder and harder to organize, and my GM itch needs scratching.

 

Just got IWD2 reloaded and modded the way I want it. Kinda forgot the mod loading order so it took a time or 3 to get everything working. Oh well, getting a bit slow upstairs I guess.

Dang cat came home with a hole in his leg again. Upset mom quite a bit, wish with over a hundred acres he could roam around he would stay away from the riverside barn cats!

 

Power is always the result of force. In the case of your cylinder pressure example, that pushes the piston down, creating a force on the connecting rod, which creates torque on the crankshaft. Force x length of crank = torque (adjusted for the angle that the force is applied).

Power isn't torque/revolution. Power is torque times RPM (and in the case of horsepower, you divide by 5252; blame James Watt again).

Haha! Yep, a bit of an "oops" moment there.

Yeah, pretty much.

 

A rather expensive "oops" moment, innit?

 

Even electrical power? Where's the force applied in a water cooker?

Anyway, like Keneth said, we all agree that the imperial system sucks

Exquisitely expensive.

They crashed another mission onto mars, too, but I don't know if it was also confusion of units in that case. The second one didn't manage to achieve orbit around mars, but crashed down on it.

 

Good point. I was thinking about mechanical power (which I am more familiar with). I don't know that there is an equivalent to "force" in electrical power. Electrical power = voltage x current; I think voltage is like a kind of pressure (more specifically, it is potential energy), and current is the rate of electron flow, but I've never been able to relate those to mechanical systems.

 

Electricity creates magnetic fields, through which force can be applied. In the case of a water boiler, though, it's just turning electrical energy into heat.

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And that force was created by a release of chemical energy. In the case of solar energy, though, the case for force being applied somewhere is a bit more difficult

 

Nah ... it took force to push the plug in the wall. That force generated the initial movement of the electrons which then accelerated to very high speeds in the presence of the electric field. The increase in speed of the electrons caused a huge (or yooge for the Canadians) increase in kinetic energy to the water cooker (after all temperature is actually a measure of kinetic energy of the system).

 

I like strawberries.

 

Ok YES I can reply to that - no kinetic energy and voltages involved ... Strawberries - they are good. I think wild strawberries are the best but they are smaller and harder to find.