Post

Michael O’Neill

@michaeloneill.org

Maybe a silly question, but is the weight of a litre of room temp/pressure air just… zero? Not the mass, the weight.

15/7/26, 7:44 am0 reposts1 like

Nicolas

@nicolasn.bsky.social

15 July

Weight is not zero, but you also have push (or lift, I don't know the name in English). Apparent weight in a balance may be zero

Phillip Broadwith

@broadwithp.bsky.social

15 July

It has mass, and weight is simply the action of gravity on that mass, so yes, 1 litre of air has a weight equal to its mass * acceleration due to gravity (m*g).

Katherine I

@chesneycat.bsky.social

15 July

No. The weight remains M x g. The net force on the air is likely zero though, assuming hydrostatic equilibrium

Katherine I

@chesneycat.bsky.social

"Weight" is an imprecise or ambiguous term. Most physics texts define it to be synonymous with the force of gravity, in which case the air parcel's weight is NOT zero. Some books define it as "what a scale reads", in which case the weight IS zero. #ITeachPhysics (1/4)

Steve-o Stonebraker

@sstoneb.bsky.social

15 July

Personally I favor the first defn and it's what most physics people use, but at least in the context of teaching introductory physics the second has some advantages too. It eliminates a lot of potentially distracting features like buoyancy (as in your case), reference frames, angles... (2/4)

Steve-o Stonebraker

@sstoneb.bsky.social

15 July

If you use "force of gravity" then you have to ALSO introduce a new "apparent weight" to give name to the sensation of being heavier/lighter while riding an elevator. Since that's such as a straightforward and familiar sensation, trying to pry the word "weight" away from it is hard. (3/4)

Steve-o Stonebraker

@sstoneb.bsky.social

15 July

Toss in the fact that relativity tells us accelerating reference frames are indistinguishable from gravity in the first place and the "what a scale reads" definition starts seeming a lot more straightforward and more empirically grounded! I think that's part of why some books go that way. (4/4)

David Harding

@groupharding.bsky.social

15 July

I would have to go with no the weight can't be zero. A liter of air will contain N2 and O2 molecules and gravity will act on those molecules giving the 1-litre box of air some (very small) weight.