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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?

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  • $\begingroup$ kilogram is the unit of mass, not weight. Weight is measured in Newton. $\endgroup$
    – CuriousOne
    Oct 2 '14 at 3:52
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    $\begingroup$ Sorry, I know about that. Was referring to everyday life. $\endgroup$
    – Jake
    Oct 2 '14 at 3:54
  • $\begingroup$ Related: physics.stackexchange.com/q/43195/2451 and links therein. $\endgroup$
    – Qmechanic
    Oct 2 '14 at 4:43
  • $\begingroup$ 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). $\endgroup$
    – CuriousOne
    Oct 2 '14 at 5:09
  • $\begingroup$ @CurioiusOne Pound could mean weight as well (lbf). $\endgroup$
    – t.c
    Oct 2 '14 at 5:39
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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...

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  • $\begingroup$ 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? $\endgroup$
    – Jake
    Oct 2 '14 at 4:20
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    $\begingroup$ @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. $\endgroup$
    – Floris
    Oct 2 '14 at 4:25
  • $\begingroup$ @Floris is that the reason why some communities use the FLT instead of the MLT dimensions? $\endgroup$
    – t.c
    Oct 2 '14 at 4:27
  • $\begingroup$ @t.c I would be speculating. Truth is, I don't know. $\endgroup$
    – Floris
    Oct 2 '14 at 4:28
  • $\begingroup$ 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? $\endgroup$
    – Nanite
    Oct 2 '14 at 6:16
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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.

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    $\begingroup$ But why create another unit when we could have used newton, can't we? $\endgroup$
    – Jake
    Oct 2 '14 at 3:59
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    $\begingroup$ 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). $\endgroup$
    – t.c
    Oct 2 '14 at 4:00
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    $\begingroup$ 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. $\endgroup$
    – anna v
    Oct 2 '14 at 4:07
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    $\begingroup$ @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. $\endgroup$
    – t.c
    Oct 2 '14 at 4:17
  • $\begingroup$ @t.c, How many percentage of people still use pounds these days? Isn't the visualizable unit now <kg>? $\endgroup$
    – Pacerier
    Jul 30 '17 at 13:32
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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.

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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.

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