Wikijunior talk:Solar System/About gravity, mass, and weight/Archive 1

Here is a list of some sources.

Wikipedia

 * Mass
 * Balance
 * Kilogram
 * Pound
 * Pound-force
 * Body mass index
 * Human weight
 * Body weight

ASTM

 * American Society for Testing and Materials, Standard for Metric Practice, E 380-79, ASTM 1979.
 * 3.4.1.2 Considerable confusion exists in the use of the term weight as a quantity to mean either force or mass. In commercial and everyday use, the term weight nearly always means mass; thus, when one speaks of a person's weight, the quantity referred to is mass. . . .  Because of the dual use of the term weight as a quantity, this term should be avoided in technical practice except under circumstances in which its meaning is completely clear.  When the term is used, it is important to know whether mass or force is intended and to use SI units properly as described in 3.4.1.1, by using kilograms for mass or newtons for force.
 * 3.4.1.3 Gravity is involved in determining mass with a balance or scale. When a standard mass is used to balance the measured mass, the effects of gravity on the two masses are equalized, but the effects of the buoyancy of air or other fluid on the two masses are generally not equalized.  When a spring scale is used, the scale reading is directly related to the force of gravity.  Spring scales graduated in mass units may be properly used if both the variation in acceleration of gravity and the buoyancy corrections are not significant in their use.
 * 3.4.1.4 The use of the same name for units of force and mass causes confusion. When the non-SI units are used, a distinction should be made between force and mass, for example, lbf to denote force in gravimetric engineering units and lb for mass.

NIST

 * U.S. National Institute of Standards and Technology, Dr. Barry N. Taylor, Guide for the Use of the International System of Units, NIST Special Publication 811,
 * Thus the SI unit of the quantity weight used in this sense is the kilogram (kg) and the verb "to weigh" means "to determine the mass of" or "to have a mass of".
 * Examples: the child's weight is 23 kg
 * the briefcase weighs 6 kg
 * Net wt. 227 g


 * [Note that the same section also defines weight in a different meaning from this one which is proper for body weight; in that other meaning, "Thus the SI unit of the quantity weight defined in this way is the newton (N)." This section also concludes with the excellent advice, "In any case, in order to avoid confusion, whenever the word "weight" is used, it should be made clear which meaning is intended."]

National Standard of Canada

 * The National Standard of Canada, CAN/CSA-Z234.1-89 Canadian Metric Practice Guide, January 1989:
 * 5.7.3 Considerable confusion exists in the use of the term "weight." In commercial and everyday use, the term "weight" nearly always means mass.  In science and technology, "weight" has primarily meant a force due to gravity.  In scientific and technical work, the term "weight" should be replaced by the term "mass" or "force," depending on the application.
 * 5.7.4 The use of the verb "to weigh" meaning "to determine the mass of," e.g., "I weighed this object and determined its mass to be 5 kg," is correct.
 * [An interesting thing to note here is a significant difference in usage of the noun forms and the verb forms.]

SAE

 * SAE TSB003, Rules for SAE use of SI (Metric) Units, May 1999:
 * 7.4.2 Considerable confusion exists in the use of the term weight as a quantity to mean either force or mass. In commercial and everyday use, the term weight nearly always means mass; thus, when one speaks of a person's weight, the quantity referred to is mass. This nontechnical use of the term weight in everyday life will probably persist. In science and technology, the term weight of a body usually meant the force that, if applied to the body, would give it an acceleration equal to the local acceleration of free fall. The adjective "local" in the phrase "local acceleration of free fall" usually meant a location on the surface of the earth. In this context, the "local acceleration of free fall" has the symbol g (commonly referred to as "acceleration of gravity"). Values of g differing by over 0.57 at various points on the earth's surface have been observed.2 In a technical context, the use of force of gravity (mass times acceleration of gravity), instead of weight with this meaning is recommended. Because the term weight is ambiguous, care should be taken to assure that the intended meaning is clear.


 * Am I reading this correctly that values of "g" (or average acceleration due to gravity on Earth at 9.8 m/s2 being the usual figure) varies by 0.57? Is that 0.57 "g's", or 0.57 m/s2 or 0.57%?  I can see some altitude variations (Mt. Everest vs. Dead Sea) or lattitude (polar vs. equitorial gravity effects).  Measuring the local gravity field was an experiment I did in High School physics class, BTW.  --Rob Horning 08:12, 14 July 2005 (UTC)


 * I'm sure that 0.57% was intended, though the pdf file only has the quoted "0.57". Actually, the normal gravity at sea level increases by more than 0.53% from the equator to the poles (9.7803267715 m/s² to 9.8321863685 m/s²), and with local anomalies from those normal values it is possible that the actual measured values at sea level could vary by 0.57%.  The average at sea level (Geodetic Reference System 1980) is 9.797644656 m/s². Throw in Mt. Chimborazo, the highest mountain on Earth in both ways relevant to this discussion, and the variation on the surface of the Earth, from there to the North Pole is more than 0.7%.
 * With that Everest and Dead Sea stuff, you remind me of the fools at Aviation Now, who tried to make a point by comparing the force of gravity on an elephant at the Denver Zoo and at the San Diego Zoo. In addition to much confusion about kilograms as units of force and using conversion factors too imprecise for their measurements and the like, they also failed to figure out the most important factor: that the acceleration of gravity at the San Diego Zoo is actually less than it is at the Denver Zoo, not the other way around.  Latitude is a more important factor than altitude is.  Metric1000 15:40, 14 July 2005 (UTC)

National Physical Laboratory (U.K.)

 * NPL FAQ
 * Weight
 * In the trading of goods, weight is taken to mean the same as mass, and is measured in kilograms. Scientifically however, it is normal to state that the weight of a body is the gravitational force acting on it and hence it should be measured in newtons, and this force depends on the local acceleration due to gravity. To add to the confusion, a weight (or weightpiece) is a calibrated mass normally made from a dense metal, and weighing is generally defined as a process for determining the mass of an object.


 * So, unfortunately, weight has three meanings and care should always be taken to appreciate which one is meant in a particular context.

U.S. Federal Standard 376B

 * Federal Standard 376B, "Preferred Metric Units for General Use by the Federal Government," January 27, 1993
 * In commercial and everyday use, and in many technical fields, the term "weiqht" is usually used as a synonym for mass. This is how 'weight" is used in most United States laws and regulations. See the note at 5.2.1 for further explanation.
 * ... [note at 5.2.1]
 * NOTE: There is ambiguity in the use of the term weight to mean either force or mass. In general usage, the term weighr nearly always means mass and this is the meaning given the term in U.S. laws and regulations.  Where the term is so used, weight is expressed in kilograms in SI. In many fields of science and technology the term weight is defined as the force of gravity acting on an object, i.e., as the product of the mass of the object and the local acceleration of gravity. Where weight is so defined, it is expressed in newtons in SI.

American National Metric Council

 * American National Metric Council, "Metric Editorial Guide," 3d ed. 1978
 * 7.1 In commercial and everyday use, the term "weight" nearly always means mass; the use of the word "weight" to mean "mass" and the word "weigh" to mean "determine the mass of" or "have a mass of" is acceptable.
 * Examples: My weight is 60 kilograms.
 * Weigh the envelope carefully.
 * The suitcase weighs 12 kilograms.


 * 7.2 Since the word "weight is used to mean both mass and force, its use should be avoided in science and technology except under circumstances in which its meaning is completely clear.  In SI, mass is measured in kilograms, and force is measured in newtons.

International definition of a pound

 * Here is the current official definition of a pound in the United States (and as you can see from the quote, in the rest of the world as well), and these are the pounds we use when we weigh ourselves "at the doctor's office". Federal Register Notice of July 1, 1959:
 * Announcement. Effective July 1, 1959, all calibrations in the U.S. customary system of weights and measures carried out by the National Bureau of Standards will continue to be based upon metric measurement standards and, except those for the U.S. Coast and Geodetic Survey as noted below, will be made in terms of the following exact equivalents and appropriate multiples and submultiples:


 * 1 yard= 0.914 4 meter


 * 1 pound (avoirdupois)= 0.453 592 37 kilogram


 * Currently, the units defined by these same equivalents, which have been designated as the International Yard and the International Pound, respectively, will be used by the National Standards Laboratories of Australia, Canada, New Zealand, South Africa, and United Kingdom; thus there will be brought about international accord on the yard and pound by the English-speaking nations of the world, in precise measurements involving these basic units.

"Measure your weight in newtons"
There are too many teachers who, being confused about how we do this in the real world, and how we should do it, confuse their students by telling them that they should routinely measure their weight in newtons.

They should not do so. The kilograms used throughout the world, including most hospitals in the United States, are the proper SI units for this weight. Note, of course, that there are no kilograms-force in SI. Metric1000 15:24, 3 Apr 2005 (UTC)

"Pounds are not units of mass"
There are hundreds of web sites, and even a few textbooks for introductory college science classes, and perhaps for junior high school or high school as well, which falsely claim that pounds are not units of mass. Metric1000 15:24, 3 Apr 2005 (UTC)

"If you weighed 100 units..."
Many teachers don't have any big problem understanding that as measurements of mass, kilograms do not vary no matter where you are (the situation is different with pounds, because few people really understand the old English units)

Note that the verb "to weigh" (as noted in the excerpts from the Canadian Standard for Metric Practice and the ANMC Guide above) is used to mean "to measure the mass of" or "to have a mass of" even many by those people who cannot bring themselves to use the historically, linguistically, and legally correct noun "weight" for the result of that measurement..

If you want people to give up their rightful, prior claim to the word "weight", then you had damn sure better give them a verb to use as well as a noun. The use of "to mass" with the meaning above remains substandard usage which grates on the ears of most people, including most physicists and chemists and the like. That's why they continue to use "to weigh" with that meaning, even if they switch words for the noun usage. Metric1000 15:24, 3 Apr 2005 (UTC)


 * This is an astronomy book, so we should use words primary in ways the astronomers themselves use them and then explain how it is different from everyday use and from the commerical and legal definitons. --StarryTG 06:54, 4 Apr 2005 (UTC)


 * About the only way astronomers ever use "weight" is in the way it was used in Cavendish's famous paper, "Weighing the Earth". Plus the occasional stepping outside their field to mangle up the proper medical sciences meaning of the word weight, as we've seen here.  Metric1000 09:15, 4 Apr 2005 (UTC)

Additions by StarryTG
I left much of the additions by StarryTG dealing with mass and gravity, but took out the two paragraphs in the "What is weight?" section. Here is what was written, and why it was inappropriate.

One of several meanings
Weight is the amount of gravitional force there is on an object with mass. The gravitional force could be from anything else with mass, such as a planet or moon. A given gravitional field would pull down on a more massive object with a larger force than a less massive object.

This is indeed one definition of the ambiguous word weight (but not quite a complete definition, because there are variations in the precise usage depending on factors such as rotation and buoyancy). However, it is not the definition of weight which is proper "in the doctor's office" for our body weight.

Remember, we are building on what the students know. This is not the "weight" they are most familiar with. They see the "net weight" listed on billions of items in our grocery stores and haredware stores and car parts stores. They see the 63 kg weight classes in Olympic boxing or wrestling or judo or whatever. They calculate their body mass index&mdash;the smarter ones doing it from height in meters and weight in kilograms, so they don't have to remember complicated conversion factors not useful in any other context. Metric1000 23:50, 4 Apr 2005 (UTC)

Misstating the reasons
Many people use the word weight to mean the same thing as mass. This works on Earth because the strength of gravity is about the same all over the world. However, we will see that the difference between weight and mass becomes noticable when we consider other planets.

We use the word "weight" with that meaning, because we've been doing so for over 1000 years. This word entered Old English over 1000 years ago, meaning the quantity measured with a balance. That quantity is mass, not force.

A thousand years ago, English-speakers used this weight as a measure of how much stuff they had, for the purpose of trade. We still measure the very same quantity for the very same purposes today, and we still quite properly and legitimately call it "weight".

A bag of sugar with a "net weight" of 2 kg would still have a net weight of 2 kg on Pluto. A man with a weight of 175 lb would still have a weight of 175 lb on Mars, using the normal and proper meaning used in the medical sciences and in sports, the primary reasons we weigh ourselves.

Yes, the change in the force due to gravity, and the unchanging mass, are both noticeable. It is still as hard to throw a baseball on Mars at the same speed you'd throw it on Earth; it will take gravity a while longer to pull it to the ground, however. Metric1000 23:50, 4 Apr 2005 (UTC)

After we got so "smart" we still measure the same thing
You might have some argument if, after we got to be so smart, we changed the quantity we measured for these purposes. But that simply did not happen.

It doesn't work ... because the strength of gravity is about the same. We simply aren't interested in how hard that pound of butter, or that 400 troy ounce bar of gold, is pressing down on our table. We want that measurement to be the same no matter what the strength of the local gravitational field. That's the way we use it; that's the way we've always done it.

We'd use the same units of measurement for that butter or that gold on the Moon or Mars as we do on Earth. We'd still calibrate our scales on the Moon or on Mars the very same way that we do on Earth&mdash;for the accurate measurement of mass in the very location in which they are used. Just stop for a minute and think about the way those scales are tested and certified in the supermarket under government supervision, or in hospitals where the government isn't so concerned about it but the doctors are, because they want the numbers from a different ward in their hospital, or from a different hospital, to be comparable&mdash;it is done the same way in either place.

In other words, we don't want to measure force, and we don't particularly care if some fools in their own jargon want to use our word "weight" with a different meaning for their own purposes. Fortunately, however, nobody was ever damn fool enough to give any physics teacher any say-so in what the term "net weight" means in commerce, a term which never refers to a force (it is not a physics term). Nor do "troy weight" and many other common terms which are always used to refer to mass, not force.

It's the same way with body weight. We aren't suddenly more fit and healthy if gravity isn't pulling on us as hard. If you are obese on Earth, you'd have pretty much the same problems related to obesity on Mars. Metric1000 23:50, 4 Apr 2005 (UTC)

There is no God-given meaning of weight
The word "mass", on the other hand, has only had this same meaning for about 275 years. So those heathen tribesmen in merry Olde England weren't making any mistake, when they were unable to discern the God-given word they were supposed to invent for this purpose, instead of the word "weight" which they did invent.

We aren't making any mistake, when we continue to use the same word with the same meaning for the same purposes today. Metric1000 23:50, 4 Apr 2005 (UTC)

Gravity varies significantly on Earth, too
Note further that while, if you use Fred Flintstone units such as kilograms-force or pounds-force, you can express the force due to gravity on Earth in numbers approximately equal to those for the corresponding measurements of mass in pounds or kilograms, the same does not hold true in the modern metric system, the International System of Units (SI). (For that matter, it also does not hold true if you limit yourself to the absolute foot-poound-second system, or if you limit yourself to the gravitational foot-pound-second system, or if you limit yourself to the gravitational inch-pound-second system, etc.)

A mass of 1.00000 lb on Earth will exert a force of from 0.99535 lbf to 1.00260 lbf, and the same of course is true if you use the obsolete kilgrams-force. But in SI, a mass of 1.00000 kg will exert a force of 9.7610 N to 9.8322 N on Earth. That certainly matters if you are weighing a 400 oz t bar of platinum, but it also matters at the precision used when a boxer is "making weight".

Even if you restrict yourself to sea level on Earth, the acceleration of gravity varies by more than 0.53%. That's easily measurable, not just in the extremes but with very small changes in location. Metric1000 23:50, 4 Apr 2005 (UTC)

This will totally miss the point
I totally hate where this article goes, and is a very strong point-of-view that weight is totally silly. Or that yout weight on another world will be the same as on the Earth. It totally lacks the NPOV requirements of Wikpedia or all of the Wikimedia projects, and as such needs to be rewritten... perhaps even from scratch.

The whole point of talking about weight on different bodies in the solar system is more "how much would I feel like I weight if I were on X?"

For example, on a videotape I got for my kids, there was a clip of Harrison Schmidt picking up a boulder on the moon. Tossing it arround and otherwise having a fun time. Mission control was a little concerned, because the rock did have quite a bit of mass (and the ability to crush,smash helmets and the suit, etc.) He also showed a very similar rock on the Earth, about the same size and mass, and simply stated there was no way he could possibly lift it up. This other clip of him in an Arizona desert even showed him trying to lift it up. He couldn't even budge it.

In short, there is a distinction between mass and weight, and it should be covered. As written (and in the rest of the articles as written so far) there doesn't appear to be a real distinction, and this discussion is getting down to the ugly fine points that is just splitting hairs. And rather silly at that.

Here is a distinction that matters to a kid if they were walking (in a spacesuit) on the Moon. They could "lift" a much heavier rock, because their arms still have the same force and energy necessary to lift things on the Earth, but on the Moon, because gravity is weaker, they can lift more. Mass is still the same, because if somebody tosses a 10 kg bag of potatoes at you, it will still hit you and knock you down just like it would here on the Earth. The mass hasn't changed, but the force necessary to lift it has dropped. Also, on the Moon you can jump up almost 16x as high because of the reduced gravity. It also takes longer to fall down, again because gravity is lower. All of this needs to be covered.

The exact opposite happens in high gravity environments, where you shouldn't even try to jump in gravity environments that high (it would break your bones if you tried), but rolling a 10kg ball over could be stopped with just as much force as it would here on the Earth.

Yes, let's be accurate, and let's talking about mass and force correctly (assuming that weight is defined as the force caused by gravity upon a mass of a certain size). These are kids we are talking about, however, and it is important to let them know that if they were walking on the moon, that they could jump much higher and lift much heavier things. The real question is how to we explain that idea and how can we describe in something they can understand what it would feel like, using just words? Rob Horning 19:41, 29 Jun 2005 (UTC)


 * So, what do you suppose happens when someone throws a 25 lb bag of potatoes at you on the Moon, compared to someone throwing a 25 lb bag of potatoes at you on the Earth? It will knock you down just like it would here on Earth.


 * What do you suppose happens were you to get hit by a 330 lb (150 kg) National Football League lineman on the moon? Or get punched by a boxer who is in the 90 kg weight class for the Olympics?  Metric1000 20:51, 2 Jul 2005 (UTC)


 * Yeah, if you get hit by a 330 lb (150 kg) lineman from the National Football League on the moon (presumably in a large domed habitat like Heinlein's "Meanace From Earth"), the poor center would be flung over the goal post as a result. Of course that 330 lb lineman can't just use his weight to drop down on somebody taking advantage of gravity, but instead has to use more of a mass concept of gaining momentum before hitting their opponent.


 * There are indeed differences, and a game of football (American or International---soccer as it were) would be very different on the Moon. The Quarterback (in American Football) wouldn't merely be doing a forward pass, but being passed forward himself in a high orbit above the field.  In many ways, it would be more like a game of Quidditch rather than the ground-based game most people are familiar with.  Real life human-powered flight could take place on the Moon, making a game of Quidditch likely to be a real sport on the Moon, broken bones and all.  As for boxing on the Moon, I think you would find it much harder to get a firm footing on the ground.  "Pro wrestling", like the WWF, would have some incredible fun on the Moon where they could do acrobatics like jumping over their opponent and landing directly on their opponent's head with their feet, or doing other more bizzare gymnastics moves.  Take a look at some of the clients that Zero G Adventure has to get an idea.  This is no longer just the stuff of NASA or government astronauts anymore.


 * This reply didn't really answer the question, which is that some sort of terminology needs to be used to describe these effects due to reduced or substantially increased gravity. So how do we describe it if you are suggesting that using kilograms as a unit of "weight" has the same meaning as on the Earth, and that 100 kg on the Moon is identical to 100 kg on the Earth?  I beg to differ that there are no differences, because there are some very substantial differences as to how you would behave, primarily because our mussels don't push against mass but rather against force.  Saying that you can run and jump like on the Earth when you are on the Moon is simply too ludicrious to even think about.--Rob Horning 07:33, 14 July 2005 (UTC)

110 pounds?
I've changed the weights from 110 pounds to 87 pounds (and corresponding metric and stone weights) to more accurately reflect those of a 10 year old boy or girl (average 10yo boy = 85, average 10yo girl = 88), according to about.com). I figured children aged 8-11 wouldn't identify with someone weighing 110 pounds, or at least it would be more relative if it was closer to their weight.--Tim Thomason 20:42, 11 October 2005 (UTC)


 * Good idea. I'll tweak it a tiny bit, however, because using 39 kg = 86 lb as a conversion factor would give you an error of only 1 part in 4630 (39.000 kg = 85.980 lb, 86.000 lb = 39.009 kg, whereas using 87 lb = 39 kg would give an error of 1 part in 85). Metric1000 10:47, 12 October 2005 (UTC)


 * I'm confused as to the purpose of the chart at all, especially since the pound is already ambiguous between a mass and weight unit so it's not really clear what we're trying to get at. &mdash; Laura Scudder | Talk 00:47, 14 October 2005 (UTC)

Leaning Tower of Pisa
Nice pictures of the balls falling from the tower, even if they would be pretty huge balls if that's to scale.

Unfortunately, the story in the text isn't true. Galileo didn't do that. He did do some experiments rolling balls down an inclined ramp, timing them with a water clock. See Galileo Galilei. Metric1000 09:21, 12 October 2005 (UTC)


 * I agree; see thread below (Rewrite?). David Kernow 04:04, 18 October 2005 (UTC)