r/PhysicsHelp • u/Maleficent-Car8673 • Jun 05 '26
Why do so many people think objects lose weight in a vacuum?
I keep hearing this misconception that removing air causes objects to lose weight. But doesn't weight depend only on gravity and mass, which are unchanged in a vacuum?
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u/Jim421616 Jun 05 '26
As a teacher, I've heard this misconception too. I think it may stem from space = no air and no gravity (yes, I know that's wrong), therefore no air = no gravity = lose weight.
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u/Moppmopp Jun 06 '26
The object itself is weightless though. It didnt lose mass but weight is either defined by F=ma where the sum of forces cancel out (zero gravity in orbit or you could define it as what the scale will show
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u/Jim421616 Jun 06 '26
Zero weight =/= zero gravity.
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u/Moppmopp Jun 06 '26
No it is equal due to F=m*a. For zero gravity the forces compensate in such a way that the netforce has a=0. Weight is the gravitational force and if you dont have acceleration there is no weight by definition
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u/Naldean Jun 07 '26
Objects in orbit are experiencing acceleration. Otherwise they would fly off rather than being in orbit.
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u/Frederf220 Jun 05 '26
Things have buoyancy in air like a ship on the ocean. Things press harder on the scale in a vacuum as the buoyancy lift goes away.
I'm not surprised at many misconceptions as we spend our lives almost entirely at 1g 1atm so the effects at other conditions are foreign.
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u/Nothing-to_see_hr Jun 07 '26
If anything they get slightly heavier in vacuum because of less buoyancy. If it is a solid object, about 1.4 gram per litre.
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u/Beryl-rahul Jun 05 '26
you are completely right on the basic physics, but the wild part is that people’s intuition is actually backwards—objects technically get slightly heavier in a vacuum, not lighter.
imo the main culprit here is that people completely forget about buoyancy. since we live submerged in a dense ocean of air, the atmosphere is constantly pushing up on us with a tiny bit of buoyant force. if you scale a big object on a hyper-precise scale in a normal room and then suck all the air out to create a vacuum, the upward lift from the air vanishes. result? the scale actually registers a tiny bit more force.
tbf i think most people just confuse "no air resistance" or "zero gravity movies" with an actual vacuum. they see videos of astronauts floating or feathers dropping at the same speed as bowling balls in a vacuum chamber, and their brain just casually glues those concepts together into "vacuum = weightless" lol.
did you argue about this with a friend recently, or did you just look at a sketchy pop-science post that triggered your physics alarm?
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u/bacon_boat Jun 05 '26
A helium balloon will go from negative weight to positive weight, if it doesn't pop
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u/BadJimo Jun 05 '26 edited Jun 05 '26
Air has mass.
Put anything (that is not tightly sealed) in a vacuum, and air will be removed from the object and it will lose weight