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What was the reason to use kilograms to measure weight (e.g. body weight, market vegetables etc.) instead of using newtons in everyday life?

Qmechanic
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Jake
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    kilogram is the unit of mass, not weight. Weight is measured in Newton. – CuriousOne Oct 02 '14 at 03:52
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    Sorry, I know about that. Was referring to everyday life. – Jake Oct 02 '14 at 03:54
  • Related: http://physics.stackexchange.com/q/43195/2451 and links therein. – Qmechanic Oct 02 '14 at 04:43
  • There is no difference in ordinary life, either. Nobody wants to buy five Newtons worth of beef, we all want half a kg (in the US that's roughly a lbs, by which we actually mean mass, not weight, it's just a misnomer). – CuriousOne Oct 02 '14 at 05:09
  • @CurioiusOne Pound could mean weight as well (lbf). – t.c Oct 02 '14 at 05:39
  • @t.c You meant lbm, or lb for short. Pounds are a unit of mass (lbm, or lb for short) and a unit of force (lbf). – David Hammen Jul 14 '15 at 00:32
  • We are simply misusing the kilogram by expressing weight as mass. But kilograms are more convenient to use vs Newtons even though Newtons would be the correct unit. Of course, weight varies with gravity where mass does not. Funny that in the foot-pound-second system we treat weight correctly (as a force) so there is an odd unit for mass called the slug (about 14.6 kg). It's there if you ever need a mass unit in the FPS system. Hope you don't. – geoO Feb 06 '17 at 06:23
  • @geoO -- Another way of looking at it: We technical people are simply being overly pedantic regarding the terms "weigh" and "weight". The Old English term "weight" was around long before the late Middle English term "mass", and both were around long before Newton. In a lay sense, "weight" and "mass" have long been synonyms. – David Hammen May 28 '17 at 01:31
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    https://en.wikipedia.org/wiki/Mass_versus_weight – Pacerier Jul 30 '17 at 13:41
  • @DavidHammen well this is a Physics Q&A forum. Sorting this out for physics learners requires accuracy. Call it pedantic but the confusion caused by sloppy language has been an annoyance to generations of physicists. And leads to errors in communication. – geoO Jan 29 '18 at 14:13
  • @geoO - This question is about "everyday life", per a comment by the OP and per the tag. The everyday life use of weight is synonymous with mass, and that usage is much older than the pedantic distinction made by physicists. IMO, it is we technical people who are at fault rather than the general public for incorrect usage. Perhaps inventing new word would have been better. But that didn't happen, so now we have a word with multiple meanings. Welcome to English. There is nothing wrong with using "weigh" as a verb meaning "assess the mass of", at least in the everyday world. – David Hammen Jan 29 '18 at 17:01
  • @DavidHammen Overly pedantic is not a criticism. This isn't English SE its Physics SE. First we have to know the correct definitions for mass and weight. Then explain how common language butchers it. Which came first is irrelevant. We are trying to explain better than the top hit at Google. – geoO Mar 22 '18 at 16:20
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    @geoO - Common languages do not butcher it. We technical people are the ones who butchered it by hijacking an existing concept with widespread use in multiple languages and then telling people that they are wrong. – David Hammen Mar 22 '18 at 16:49
  • @DavidHammen Every technical term (and concepts such as "mass") had some plain language origin even if only a latin or greek root. Science provides order to the lingual chaos. Getting definitions right is the first order of business in science and medicine for example, where mistakes can really cost you. It's like when people say "theory" when they mean "hypothesis," or "turn it left" when they mean "counter-clockwise." That just will not do when trying to get things done. Technical people do not butcher a language but provide precise definitions. At some point you need to set a meaning! – geoO Mar 23 '18 at 18:09

4 Answers4

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The problem is that while mass is the same everywhere on earth, weight is not - it can vary as much as 0.7% from the North Pole (heavy) to the mountains of Peru (light). This is in part caused by the rotation of the earth, and in part by the fact that the earth's surface is not (quite) a sphere.

When you are interested in "how much" of something there is - say, a bag of sugar - you really don't care about the local force of gravity on the bag: you want to know how many cups of coffee you can sweeten with it. Enter the kilogram.

If I calibrate scales using a reference weight, they will indicate (at that location) the amount of mass present in a sample relative to the calibration (reference). So if I have a 1 kg calibration weight, it might read 9.81 N in one place, and 9.78 N in another place; but if I put the reference weight on the scales and then say "if you feel this force, call it 1 kg" - that is what I get. You can now express relative weights as a ratio to the reference.

All I need to do when I move to Jamaica (would that I could…) is recalibrate my scales - and my coffee will taste just as sweet as before. Well - with Blue Mountain I might not need sugar but that's another story.

So there it is. We use the kilogram because it is a more useful metric in "daily life". The only time we care about weight is when we're about to snap the cables in the elevator (too much sweetened coffee?) or have some other engineering task where we care about the actual force of gravity (as opposed to the quantity of material).

So why don't we call it "mass"? Well, according to http://www.etymonline.com/index.php?term=weigh, "weight" is a very old word,

The original sense was of motion, which led to that of lifting, then to that of "measure the weight of." The older sense of "lift, carry" survives in the nautical phrase weigh anchor.

Before Newton, the concept of inertia didn't exist; so the distinction between mass and weight made no sense when the word was first introduced. And we stuck with it...

Floris
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  • hmm... I definitely agree as to the "how much" we are measuring, but if people are free to calibrate their scales to the local gravity, then why don't why just call it mass rather than weight or kg-f? – Jake Oct 02 '14 at 04:20
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    @Jake - we called it "weight" long before people made the distinction between mass and weight: according to the etymological dictionary, "The original sense was of motion, which led to that of lifting, then to that of "measure the weight of." The older sense of "lift, carry" survives in the nautical phrase weigh anchor." and it is a very old word. The concept of force is much more recent than the concept of weight. – Floris Oct 02 '14 at 04:25
  • @Floris is that the reason why some communities use the FLT instead of the MLT dimensions? – t.c Oct 02 '14 at 04:27
  • @t.c I would be speculating. Truth is, I don't know. – Floris Oct 02 '14 at 04:28
  • Huh... I've been doing physics for a long time now and I just realized that the calibration of a spring-based scale is gravity dependent.

    I guess this is not a big deal for most things (who cares if your body mass is in error by +- 1%). But how do manufacturers of ultra-accurate electronic scales manage this problem? Do they have a reference mass inside for compensation?

     – Nanite Oct 02 '14 at 06:16
  • @Nanite - I recently bought a cheap 5-digit scale (about $30 on Amazon). It shipped with two 10.000 g calibration weights and instructions for how to use them... Any time you want accuracy better than 1% you have to be prepared for some calibration effort (although it is sometimes possible to hide the effort from the user). But a mg scale with 20g full scale capacity needs a reference mass. – Floris Oct 02 '14 at 11:18
  • @Floris, What's with the Jamaica reference? – Pacerier Jul 30 '17 at 13:27
  • @Pacerier I like Jamaica - and it is at a very different latitude than where I live. Good enough? – Floris Jul 30 '17 at 13:30
  • @Floris, Hmm, "latitude" meaning temperature? Also, re "Peru", why? It's the furthest to Earth core? – Pacerier Jul 30 '17 at 13:43
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    Peru: I think it is a combination of (1) close to the equator, (2) high altitude, and possibly (3) lower density below the surface? Not sure about the third point. Latitude is just a measure of how close to the equator you are. – Floris Jul 30 '17 at 17:11
  • @Floris latitude is a measure of how far from the equator you are. – phoog Feb 07 '20 at 17:52
  • I don't think this is correct. If you buy 1 Kg of apples you are actually buying 1*g Newton of apples where g is the gravitational acceleration in your location. No 'everyday' scale actually measure mass (as you also point out). Instead they all measure a force. Well, some scales (the very old ones with two arms..) actually measure the ratio with respect to a reference mass, as you say. But only those kind – lcv Feb 07 '20 at 18:28
  • @lcv any scale that is calibrated in kg should get calibrated with a reference mass and the measurement ends up being relative to that mass. The old “balance” does it directly, every other scale does it by comparison to the reference. Only a scale calibrated in Newtons would be the exception. Anyone who claims to measure kg when they are measuring kg*(local g) is misleading. – Floris Feb 07 '20 at 22:13
  • No that's the point, scales are calibrated with a reference force, not mass. Most such scale use piezo that end up exploiting some version of hooks law. So they measure a reference force to the location they were built. If you bring them to another location they will be off. If you use and old balance you don't have this problem because both forces (also the reference one) are proportional to mass. – lcv Feb 07 '20 at 22:37
  • @lcv I don’t know where you get the idea that commercial scales are calibrated with a force. See for example https://www.precisionsolutionsinc.com/scale-calibration-guide/ – Floris Feb 07 '20 at 23:45
  • The fact that they put a mass on the scale doesn't mean anything since those scales simply measure a force. For electrical scales you reach equilibrium when $m g = k \Delta x$. If you move the scale to a different location $ g $ will change and you will get equilibrium with a different mass. If you use an old balance you reach equilibrium when $m_1 g = m_2 g$ i.e. when $m_1=m_2$. If you change location you will get a different $ g$ but it will be the same on both sides of the equation. – lcv Feb 08 '20 at 06:03
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    @lcv which is why you need to calibrate a scale with a mass, at a location. Moving the scale invalidates the calibration. At least that is the point I tried to make in my answer. – Floris Feb 08 '20 at 19:16
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Yes. When we use kilograms to measure weight, we are actually referring to $kg_f$ or kilogram-force.

http://en.wikipedia.org/wiki/Kilogram-force

From Wikipedia: One kilogram-force is equal to the magnitude of the force exerted by one kilogram of mass in a 9.80665 m/s2 gravitational field.

In other words, the weight(force) of one kg is equal to one kgf, or 9.8N.

t.c
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    But why create another unit when we could have used newton, can't we? – Jake Oct 02 '14 at 03:59
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    If we use newton, a 50kg man would weigh 490N, which is a hassle in everyday's calculation. Imagine going to the market and buying 4.9 Newtons of apples. However, if we use kgf, a 50kg man would weigh 50kgf, which is much more convenient. Most people, when referring to the kgf, simply calls it kg (which is technically wrong, since kg is a unit of mass not force). – t.c Oct 02 '14 at 04:00
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    Actually because a kilogram is about the order of magnitude that people were using to measure produce, the oke, very spread in the middle east is about factor of two fto the kilogram. The pound is about half a kilogram.It is an order of magnitude that must come by how much food feeds a family or similar arguments, a good weight in barter. Newtons are a factor of ten. – anna v Oct 02 '14 at 04:07
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    @annav Probably due to reluctance to change. The slug and poundal has been invented but people still use the popular pound-force and pound-mass. For kgf, its the other way round - it is easier to invent kgf than to change the reference to weight in everyday life to Newtons. – t.c Oct 02 '14 at 04:17
  • @t.c, How many percentage of people still use pounds these days? Isn't the visualizable unit now ? – Pacerier Jul 30 '17 at 13:32
  • Actually the kgf represents the gravitational force on a kg of mass at a very specific locus only. On the other hand the kg can be defined anywhere by reference to an object with known mass (“a kg is that mass which balances my scales when I put this other kg on the other side”) – Floris Oct 29 '19 at 01:15
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So what I understand from reading the other answers is this:

Here on earth: 1 Kilogram of lettuce: Mass-> 1 Kg, Weight-> 1 Kgf

In another hypotetical planet where gravity is half of earth: 1 Kilogram of lettuce: Mass-> 1 Kg, Weight-> 0.5 Kgf

Since there is no practical easy way to measure mass, in everyday life we use the kilogram as a unit of weight assuming that the gravitational field is fairly constant around earth. However scales have to be calibrated locally to compensate the slight gravitational field variation in different places.

Camilo
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Why do we use kilograms instead of newtons to measure weight in everyday life?

The other answers are all very good, but the everyday life answer is that it's because gravity is constant. Yes, I know, it's not really constant, but for most everyday life purposes, it is.

Because gravity is constant, we don't need to distinguish between mass and weight.

In everyday life. For most purposes.

phoog
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