negative weight

[ed2288]

if you tie a balloon of helium to some scales, the reading will go down - so how come a mole of helium weighs 40mN?

[gen2]

You have to weigh it in a vacuum to get its true weight. It only registers a nenegative force in air because it displaces a greater mass of air (nitrogen/oxygen) and therefore tries to float .

[Peter Pedant]

It will all be under statics, archimedes and upthrust

Imagine a boon filled with air, it dont move. SO if it weighs 50 mN, [draw a circle with a downward arrow and 50 by it] then there has to be an arrow pointing up ward, with 50 on it, otherwise by Newton second law, the boon would sink

Same volume, filled with helium, Centre of circle has 40 iin it (see your question), upward arrow is still 50 and zippedy doo dah, younow have an upward force of 10 lifting said helium up.

der daaah. Archimedes proably thought of somehtinng like this in the bath....

[bernardo]

I'm curious: What is a "boon"? What do "zippedy doo dah" and "der daaah" mean? Is it one of the factions of Marxist-Leninist dialectics?

[Catso]

It has a weight, of course. It's just less than the same amount of air, so it tries to float in air.

You wouldn't say 'how come wood has weight, when it can float in water'

(And I'd have done 'der-daaah' as 'ta daahhh')

[Peter Pedant]

I was intrigued as the question uses the correct unit for weight (Newtons) and not those of mass (kg)

Something I do not do automatically - I weigh 100 kg and not 900 N , and then asks an amazingly simple question like what archimedes couldhave answered.....