A gas in a Box

No--it's buoyancy. The box has volume that displaces air. The gas in the box, depending upon its density may be less than the air the box displaces or greater than the air it displaces.

Rap

You are correct. The weight of the sealed combination depends on the density of the surrounding medium. But it equally depends on the location of the weighing whether that be on earth or elsewhere!

Then it is not weight. It is mass.

Please clarify

Weight is a force. Mass is an amount of matter. The force of a unit of mass one the earth (1 gravity or 32.2 ft/s^2 acceleration) is called the weight. If that mass is removed from that gravity (say the moon which is about 1/6 th g) the weight of that unit mass is 1/6th the weight on earth. The mass is the same.

Newton expressed this algebraically as F=ma--force equals the product of the mass and the acceleration.

Rap

My point was that weight is concerned specifically with gravitational force which is proportional to the product of attracting masses divided by the distance between their mathematical "centers". Hence "weight" even on earth varies depending on whether you are at the poles or the equator due to variation in earth's radius. (I believe differential centrifugal force also plays a part.)

(NB Newton used the term "rate of change of momentum" not " mass x acceleration". This has significance when mass splits on impact for example)

CORRECTION"......divided by the square of the distances between...."

Correct F=d/dt(mv).

But in bulk mass doesn't change (even with fractioning mass) until you approach relativistic velocities or you're dealing with reactions (e.g. rocket impulse), or (more interestingly) when when the mass is changing direction (leading to an explaination of centrifugal force).

However, in straight lines for all practical purposes (using another of Newton's discoveries)

F=d/dt(mv)=m dv/dt =ma.

BTW the gravitational attraction is modeled by Fg=G*m1*m2/d^2
Fg is force of gravity
Where G is the gravitational constant
m1 and m2 are two masses--
d is distance between the two masses

Centrifigul force is modeled by
Fc=m2*v^2/d
m2 is orbiting mass
v is velocity
d is orbiting distance (radius)

So for an earth orbiting body Fg=Fc

G*me*m2/d^2=m2*v^2/d
As rotating mass is on both sides it cancels out
G*me/d=v^2
v=sqrt(G*me/d)
As G*me is a constant (K) then
v=sqrt(K/d)

as v is a straight line the orbit circumference is 2*pi*d
so v=2*pi*d/Ot
where Ot is the time of a single orbit
2*pi*d/Ot=sqrt(K/d)
and
Ot=2*pi*d/sqrt(K/d)
Ot^2=(2*pi)^2*d^2/(K/d)=(2*pi)^2*d^3/K
so
Ot=2*pi*sqrt(d^3/K)
Which agrees with Kepler's Laws (assumming a perfectly circular orbit.

Rap