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How do you write the possessive for an abbreviation of a regular plural noun, when the plural 's' is not present in the abbreviation?

I want to write "ten kilograms' weight" in a scientific context where the abbreviation kg is used for the plural word kilograms.

How do I write this? Is it "10 kg's weight", or "10 kg' weight", or something else?

Thanks!

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    If you're doing it in a scientific context, it should just be 10 kg. No "weight", no "s".
    – Peter Shor
    Commented Nov 19, 2018 at 2:35
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    By "ten kilograms' weight" I specifically mean the weight (as measured in newtons) possessed by a mass of ten kilograms. The term "kilogram-force" does exist to describe this, for which there is an abbreviation "kgf"; but I think that kgf is an actual unit that assumes a very specific value for the gravitational field strength. I just want to communicate "the weight of 10 kg of mass".
    – Julian Newman
    Commented Nov 19, 2018 at 2:55
  • You only need to use the unit ("kg") in all contexts. For any disambiguation or for improving clarity, the sentence may need to rephrased, which is a different issue. HTH.
    – Kris
    Commented Nov 19, 2018 at 8:05
  • I think "a/the weight of 10 kg" would be fine. Your audience will understand the difference between mass and weight, so I don't think there's any need to use the awkward "the weight of a 10 kg mass."
    – Peter Shor
    Commented Nov 19, 2018 at 13:43

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You don’t use the possessive with scientific units. In fact, you probably don’t have to use the possessive with any units, although there are figures of speech that appear to do so, e.g. a day’s pay for a day’s work.

In any event, the abbreviations for SI units always appear in their simple form directly after the number they apply to. Plural and possessive forms are not used.

BTW, it’s 10kg of mass, not weight. In ordinary life we use the term weight more freely, so it’s perfectly okay to say that a person “weighs 70kg”. Even in science and engineering journals, you can sometimes see expressions like this in the description of everyday objects.

However, if you are writing for a technical audience, you should follow the appropriate SI style.

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  • As explained in my reply to Peter Shor's comment, I actually want to communication "the weight possessed by a mass of 10 kg". So in this context, are you saying that "ten kilograms' weight" would be written "10 kg weight" (i.e. no apostrophe or s), or are you saying that it is simply not permitted in the scientific parlance to write (in any abbreviated or full form) the phrase "ten kilograms' weight"?
    – Julian Newman
    Commented Nov 19, 2018 at 3:09
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    The construction you use in your response to Peter Shore, "the weight of 10 kg of mass", might be the best option. I agree with each of the other posters: a unit of measure gets no possessive. The distinction you are trying to make is not carried by an apostrophe. Think of it this way: units simply have no property rights, nor trouser pockets in which to put things.
    – Kay V
    Commented Nov 19, 2018 at 3:39
  • @KayV: Thanks for your recommendation about the option. As for the rest of your comment: when you say "gets no possessive", I can't tell whether you mean that if it is necessary to write "ten kilograms' weight" in abbreviated form then this becomes "10 kg weight" with no apostrophe or s, or whether you mean that there simply is no valid way to write "ten kilograms' weight" within the standard units system.
    – Julian Newman
    Commented Nov 19, 2018 at 21:23
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    Thanks for pointing out the ambiguity, @JulianNewman. To clarify: there simply is no valid way to add a possessive to a unit of measure.
    – Kay V
    Commented Nov 23, 2018 at 21:14
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You wouldn't use the possessive like this. It's not usual to use the unit name kilogram as a noun meaning "a mass of 1 kilogram". So the issue of contraction is really a distraction here: even "ten kilograms' weight" doesn't look right.

If you want to be pedantically precise, you could say "the weight of a 10-kg mass".

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  • To clarify - just in case you have misunderstood me (although I don't think you have) - I am not trying to refer to an actual object with mass 10 kg. I am simply trying to say "the amount of force equal to the weight possessed by 10 kg". In the past, this amount of force was called "10 kilograms-force", but the term is now somewhat outdated and also makes reference to a specific convention for the value of gravitational field strength. I find it hard to believe that there is no concise convention for communicating the amount of force equal to the weight of x kg of mass.
    – Julian Newman
    Commented Nov 19, 2018 at 3:21
  • I see. Well, "a 10-kg mass" doesn't have to refer to any specific object, it could just be used as a way of referring to how any object with a mass of 10 kgs would behave.
    – herisson
    Commented Nov 19, 2018 at 3:25
  • @JulianNewman Weight is the force exerted on an object in a gravitational field. An airplane flies because its engines provide thrust (measured in newtons). The resistance of the air against the movement of the aircraft creates drag (also in newtons). When the thrust exceeds the drag, the mass of the aircraft is accelerated forward until it reaches a velocity where the thrust and the drag come into balance, since drag increases with velocity. At that point, what matters is the lift versus the weight, the latter being the mass of the aircraft times the acceleration due to gravity, 9.81 m/s^2.
    – user205876
    Commented Nov 19, 2018 at 4:20

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