rmfnla

You are correct; my bad.

juicemouse

Huh, I didn't know that. So you would say that a person who weighs 150 lb would have a mass of 68 kilos, then? I guess I was thrown because I thought you referred to it as "how much someone weighs" not "what is their mass", don't you? The number will be 68 regardless, but if you refer to it as the person's weight, then I stand by my original statement that weight is a force and the appropriate units for that (under the system I described) would be kgf, not kgm. I know everyone just use kilograms or kilos, but I think those kilos correspond to kilograms of force, not of mass. Thanks for correcting me though, as you are the authority on this matter.

juicemouse

Sorry for getting my boxers all in a knot. I'm just really starting to hate that stereotype.

moxfyre

I agree... it's fairly pedantic to distinguish between mass and weight on a daily basis, since everywhere except in outer space g is nearly constant. That's why we freely use lb as a unit of mass or kg as a unit of weight, since the conversion factor never changes.

On the other hand, I think it's foolish to use units like "kgf" for anything other than weight. The kilogram unit has no obvious, natural translation into a unit of force except when that force is weight. In a torque specification for a bolt, the force is an arbitrary mechanical force applied to a lever. The "kgf" unit therefore causes a great deal of confusion, as we've seen in this thread. I dunno why engineers use "kgf" instead of "9.86 Newtons."

CdCf

We refer to it as weight, yes. No argument there.
But what we call weight is really the mass, of course.
Those who need to know the difference generally do.

juicemouse

I think we agree on everything but jargon. The reason I'm insistant that a weight cannot ever be measured in units of mass, though, is because as soon as the gravitational constant changes, you'd measure a different mass. This is the whole idea behind kgf, to keep it in units of force. I think your statement that "those who need to know the difference generally do" is spot on, but I think it makes it less confusing if you just always treat weight as a measurement of force.

juicemouse

Unless I'm missing something, we don't commonly use lb as a unit of mass in anything but engineering. It's not that we don't distinguish between mass and weight on a daily basis, it's that we don't even consider mass on a daily basis.

I understand your concern that describing a force in units of kgf is "foolish". The situation that engineers sometimes do this in is when that force is part of a system that mainly has forces due to gravity. The force itself doesn't have to be a weight, but if there are a whole lot of other weights in the same system, it makes it easier to just ignore the gravitational acceleration in the problem and treat all forces in units of kgf, rather than figuring out how many Newtons each mass weighs. In the case of the seat post binder bolt, I agree that it would be much more clear if it were labled in units of Newton-meters (N-m), since no part of that system is really affected by the force of gravity. I could see it if torque wrenches were calibrated in those units, but I know my own is calibrated in N-m (as well as lb-ft). There is a time and a place for everything, and this ain't the place for kgf.

CdCf

Even worse is the common practice here of using kg as a unit of pressure!

moxfyre

So... what is a "kg" of pressure? Is it = 1 kg * g / cm^2? Which would be approximately 10 Pa?

juicemouse

Wow. I won't go there.

rmfnla

That was part of what I had in mind, but I tend to get a little sloppy with my posting at times.

I'm not sure which freshman classes are more important here, Physics or Creative Writing..!

CdCf

You're partly correct, but I think you've forgot what 1 Pa is.
1 Pa = 1 N / m^2, not 1 Pa = 1 N / cm^2.
The difference between 1 cm^2 and 1 m^2 is 10 000.

So, 1 "kg" of pressure is really around 98 kPa, or essentially 1 bar, or 1 atmosphere.

moxfyre

Thanks, that is confusing indeed

What I meant was, of course, 10 Hecta-Pa

dedhed

Get used to it if your going to be an engineer. Until you've been at it a while or in the same place to prove to the others you know your ***** thats just how it works. Look on the bright side, at least you have girls in Engineering now, pretty few and far between 25 years ago.

juicemouse

The bright side of what? Having an engineering background? What a bunch of crap.

cerewa

i still can't figure out what a kilogram of pressure is supposed to be. a kilogram per square inch?

i think just to "torque" people off i'm going to start talking about my bicycle tire pressure in kilograms per square inch.

CdCf

1 kg of pressure = 1 kg * ~9.81 / cm^2

moxfyre

Who comes up with units like "kilogram of force" and "kilogram of pressure"???? (Excluding the case of weight, where kgf is fairly intuitive)

The former is 9.81 N, the latter is 98.1 kPa (nearly 1 bar)... "10 N" or "100 kPa" would be a decent approximation for both. Who decided that using a wrong unit would be easier?

juicemouse

Impossible to say. Having not lived in Europe, I'd be interested to know how widespread the use of kg as a unit of both force and pressure is. Are Newtons and Pascals anywhere to be found? I will say, though, that as soon as you get used to any unit system, any other system seems odd. I'd venture to say that those who use kilograms as a unit of both force and pressure find it quite natural. What makes using kg in these situations "wrong"? Do you think using lbs. and psi. is any "better"?

CdCf

Even though the unit of length is wrong, psi is at least a unit with all the ingredients in the name itself...

moxfyre

Yeah, I think lbs and psi is better, although I'm a pro-metric kind of guy. Why? Because "pound" is explicitly a unit of force, and therefore "pound per square inch" is explicitly a unit of pressure.

With things like "kgf" and "kilogram of pressure", there are hidden conversion factors (g and g/cm^2) that must be introduced in order to get the right units to come out!

For example, in my above post I asked what a kilogram of pressure is. Is it kg*g/cm^2, or is it kg*g/m^2??? I would have had no way of knowing which was correct without looking it up in some kind of engineering reference. The idea of "kilogram of pressure" is confusing, because it suggests one kind of unit (namely mass) but actually signifies another (namely pressure). On the hand, if you hear "Pascal" and don't know what a "Pascal" is, you won't get confused by a false association.

CdCf

I do know my tyres need around 0.6 MPa in them.

moxfyre

Oh, you mean 6.12 kilograms of pressure, right?

juicemouse

I agree with you about kilograms being used as a unit of pressure, but there aren't any "hidden" conversion factors in the kgf unit. That's the whole point, to avoid having to use a conversion factor to get from mass to weight in engineering-type equations. But I assume you mean that you shouldn't have to convert between kgf and Newtons, and I completely agree. I think that unless most of the world decides that they want to use kgf as the default standard unit of force instead of Newtons (and I'm not sure that they haven't decided this already), the kgf unit should stay in engineering equations only and shouldn't appear on consumer products like the OP's seat post clamp. I think it's a bad idea to use one unit for weight (kg) and a different unit for all forces except for weight (N) [see comment below]. I think it's a bad idea to use one unit (kg) to mean three different things in three different situations. I think that if we're going to measure weight in kilograms, we should use the kgf unit instead of simply kg, for clarity.

[This, by the way, is the reason I suspect kg is being used for forces and pressures in Europe. People get used to seeing kg as a unit of force when they weigh stuff, so they start using it for other forces because they're accustomed to using it.]

Having said that, it is what it is, and I don't know about you but I'm not going to war over it. Complete standardization (which is what you're asking for) sure would be nice, but it hasn't happened yet and doesn't look real likely in the near future. That's what I meant with my lbs and psi reference (though I agree with your point too), that the US system requires so many damn conversion factors but we're not going to standardize and go to SI (metric) for the simple reason that everyone's gotten accustomed to working within the US system. As an engineer, my responsibility will be to deliver products labeled in units that are most commonly used by the target audience. That may mean kg or kgf in European countries, I'm not sure. What this boils down to is that you'd better study your conversion factors.

juicemouse

And I direct you to your own post:


People measure weight all the time, but seldom measure pressure or force. What do you expect them to do when they get accustomed to using kg as a unit of force?