This isn't really about astronomy as in "star gazing", but it's sort-of about space and the universe and physics, and I'm posting here because I think that the Bad Astronomer and I are on the same side of an argument, but not necessarily the right side.

The owner of http://ourworld.compuserve.com/homep...ard/weight.htm dropped into my favourite Usenet newsgroup to tell me off for repeating what he considers to be a fallacy - that pounds measure weight, force,
gravity, whereas kilogrammes measure mass. He says that they're _both_ mass, and, furthermore, that "weight" usually means mass anyway.

I presume that everyone here appreciates the necessary distinctions between physical quantities; when sent to places where the force of gravity is different to the average Earth sea-level altitude gravity, objects "weigh" less (or more) whilst having the same mass.

So anyway, I thought, "Who do I respect on the Internet, that knows about stuff like this?" So, Doctor, here I am [img]/phpBB/images/smiles/icon_wink.gif[/img]

On http://www.badastronomy.com/bad/misc/planets.html our host takes the line pounds = weight, but admits having carelessly written kilograms = weight in a previous article.

Well, then, who's right? Gene Nygaard doesn't come over as a guy who's arguing without checking his facts, or without taking note of contrary evidence. But I don't know how to evaluate the question.

I do know that pounds as a scientific measurement are dead, dead, dead, baby, even before someone used them to fly a spaceprobe smack into the side of a certain planet, and defending pounds against an allegedly false charge that they measure the wrong quantity is not going to change that [img]/phpBB/images/smiles/icon_wink.gif[/img]

Yep, pounds are a unit of weight. The British unit of mass is called the slug. I teach high school phyiscs and every year when I tell my students this fact, they all want to go the library and look it up.

They always come back amazed that the dictionary defines the slug as a unit of mass!

Rob

I would say that Gene's wrong . . . how many times haven't you heard about how a 180 lb astronaut would only weigh 20 lb on the moon or something like that?

The fact that the NIST has established a definition of "pound" that's a specific fraction of a kilogram has nothing to do with how the word is often used.

One could say that the statement "pounds only measure force" is wrong because a "pound" is also a public kennel for stray dogs, and it'd have just about as much relevence . . .

IMHO, of course. [img]/phpBB/images/smiles/icon_smile.gif[/img]

__________________
SeanF

"Ask to understand, but don't challenge unless you have the knowledge."--NEOWatcher

The contents of this post are ©2008 by SeanF and may not be copied or retransmitted in any form without the express written consent of SeanF

Surely:

Mass = kg
Weight = mass x g (in Newtons)

When dealing with a scientific context.

Pounds are then just the Imperial equivalent of Kg, making them mass.

__________________
Up the Imps!

No, pounds correspond to Newtons : both are units of force.

kilograms correspond to slugs, units of mass.

The confusion comes from the commonly cited conversion factor 1kg = 2.2 lbs. This conversion factor ONLY applies where the acceleration due to gravity is 9.8m/s^2.

1 kg = .0685 slugs
1 Newton = .225 lbs

Rob

There is, or was, a little used system of units in which the unit of mass is pounds and the units of force is the poundal.

In this system, 1 poundal of force gives an acceleration of 1 ft/sec^2 to 1 pound of mass.

<font size=-1>[ This Message was edited by: David Simmons on 2002-02-22 18:34 ]</font>

The pound besides being a monetary unit is also a unit of force and a unit of mass.

The pound-mass is actually older than the pound-force and is the basis for the pound-force. The weight of one pound-mass is one pound-force (at sea level). Or if you prefer conversions: one pound-mass = 1/32.17 slugs.

For reference try these web sites:
Eric Weisstein's World of Physics
Gene's Force v. Mass Page

Hope this helps,


<font size=-1>[ This Message was edited by: Wiley on 2002-02-22 10:19 ]</font>

David,

You snuck in your post whilst I was writing mine. But you gave the definition of a slug not a pound-mass. One pound-force will accelerate 1 slug at 1 ft/s^2. One pound-force will accelerate 1 pound-mass at g.

This happens with kilograms too.
I remember learning about kilograms as a measure of mass, and kilograms-force as a measure of weight.
On the surface of the Earth, they are *numerically* identical, but we wrote kg for kilograms and kgf for kilograms-force (kg m/s^2).
kilograms-force can be converted into newtons.

<font size=-1>[ Corrected typing mistake and units.]</font>

<font size=-1>[ Oops!... Corrected units again.]</font>

<font size=-1>[ Correcting the units again... Darn!]</font>

<font size=-1>[ This Message was edited by: informant on 2002-02-22 15:36 ]</font>

As said, in general, pound (lb) refers to weight, not mass. But there is a valid distinction between pound-mass (lb-m) and pound-force (lb-f)...but you need to specify this otherwise "pounds" refers to weight only.

In scientific and engineering uses, there is strict adherence to the mass vs. weight units (lb-m/lb-f or kg/newton). But as HaleBopp said the common (marketplace) usage confuses the issue. As far as I know, vendors in countries that use metric-only do not sell their wares by "the Newton" but per kg. Do I have that right?

I am not an expert on units. However, after reading the page in question, I think he is wrong. He says the pound is a measure of mass, and that is simply not true. Mass is an intrinsic property of matter, and will be the same no matter where in the Universe you are. Your weight, however, changes with the local gravitational force.

A more careful definition of weight might be "the response of mass to gravity", as opposed to simply calling it a force. But as far as I know (and again, I am not an expert and am willing to listen to cogent arguments on this) weight has the same units as force, and not mass. You can convert between them using a number (like 1 kg = 2.2 pounds) only when in a constant gravity, like the surface of the Earth.

I found his webpage to be confusing, too. I am not sure exactly what he is trying to say, since his basic thesis isn't stated simply on it. I have a few pages like that myself. [img]/phpBB/images/smiles/icon_wink.gif[/img]

You have it right. But on the surface of the Earth it's the same thing to use mass or weight. They are numerically identical.
It's easier to say "kilogram" than to say "kilogram-force" (and who bothers with forces in a marketplace?)
Also, as Wiley said, pounds/kilograms are older as a measure of mass (=using a scale).

(I was replying to Another Phobos, not the BA.)

<font size=-1>[Added Wiley's name.]</font>

<font size=-1>[ This Message was edited by: informant on 2002-02-22 10:57 ]</font>

Guys,

In all our excitement, we have forgotten to welcome Robert to the board.

Welcome, Robert and happy postings.

Let me ask you all a really stupid question. I have been reading this thread with great interest as I realize that these questions/arguments have been around probably as long as the time when the units were devised.

Since we all know w = mg, w becomes a conspicuous quantity, one that we can measure, just like lbs. Now, in the absence of gravity (as in microgravity conditions), how do we even measure the "mass" of an object? In this case, weight becomes a relative term and mass becomes an absolute. How do we measure this absolute without a frame of reference - in this case some g value?

Sorry, I din't mean to digress from what is being discussed.

[img]/phpBB/images/smiles/icon_confused.gif[/img]

Hey, you never welcomed me!! Just kidding.
[img]/phpBB/images/smiles/icon_cry.gif[/img]

Not a stupid question at all. And the short answer to how do you measure mass in a microgravtiy situation is: since f=ma,

1) apply a known force and measure its acceleration. (Spring occilation since spring constant is known)

2) apply a know acceleration and measure its force (centrifuge)

In the presence of a gravity that is unknown you can measure mass using a balance scale. Comparing your test object with standardized mass objects.

[Edit] I wasn't very clear first time



<font size=-1>[ This Message was edited by: amstrad on 2002-02-22 12:25 ]</font>

Interesting...I have never heard of the pound-mass, although it would be valid (if unconventional) unit.

You an also measure mass using a device called an inetial balance. It's hard to describe, but it is a tray that oscillates parallel to the ground. Ther peiod of oscillation is related to the mass in the tray. It is a harmonic oscillator, so the equation is similar to that of a simple pendulum or mass on a spring. Once you establish the constant of proportionality using standard masses, the mass of an unknown object can be found that is indepenent of gravity.

It is a pretty cool intro physics lab.

Rob

Apologies, SiriMurthy, I missed the thread of your initial post. So a hearty belated welcome to you.

Well, in all systems of units for force, mass and acceleration, one unit of the force will give 1 unit of the mass an acceleration of one unit of distance/one unit^2 of time.

1 pound force gives 1 ft/sec^2 to 1 slug.
1 poundal force gives 1 ft/sec^2 to 1 pound.
1 Newton force gives 1 meter/sec^2 to 1 kg.
1 dyne force give 1 cm/sec^2 to 1 gm.

one anything gives 1 anything/anything^2 to 1 anything. As long as the system is consistent, it doesn't matter what units you use.

etc., etc., etc.

In the poundal, pound, ft., sec. system, 32.2 poundals of force gives 1 g acceleration to 1 pound of mass.

Mea culpa. I read pound when you actually wrote poundal. To quote George Jefferson, "You 'ight, Wheezy."