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The weighing scales in my kitchen are currently showing 0 Kg. However, there is a column of air between the scales and the ceiling that is presumably exerting pressure on them. If I were to place those scales in a vacuum, would those scales show a negative weight?

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up vote 8 down vote accepted

They would show only a very tiny negative positive weight. A typical kitchen scale has air above and below the weighing pan, and the pressure is very nearly the same because the thickness of the weighing pan is millimeters and the height of the atmosphere is a million times that. The net force of the pressure is quite small - equal to the weight of the air it displaces, and air is about 10^3 or 10^4 times less dense than typical solid things. So if your pan weights a few hundred grams, the scale would read a few tenths of a gram in a vacuum.

EDIT: When there's atmosphere, there's a buoyant force pointing up. When we lose atmosphere, there's more force down and the tared scale now reads positive. Originally I got the buoyant force correct but the reading on the scale backwards.

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But you're forgetting about the weight of the weighing pan itself. It your pan weighs a few hundred grams, but the scale reads 0 on Earth, then it would read negative a few hundred grams in free fall. –  Keenan Pepper Nov 17 '10 at 23:50
    
@Keenan: good point, I forgot about that as well. –  David Z Nov 18 '10 at 0:00
4  
No I didn't forget. The question said vacuum, not weightless. –  Mark Eichenlaub Nov 18 '10 at 1:16
    
d'oh, silly me ;-) you're right, I got caught up in the discussion and completely misread the question. –  David Z Nov 18 '10 at 2:47
    
Oh, well shut my mouth. –  Keenan Pepper Nov 18 '10 at 15:41
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It would vary from weighing scale to weighing scale. In my experience, most of the scales subtract the weight of the "tare", which is the thing that you put your "thing to be weighed on". The scales which use this system will show a noticeable amount of negative weight.

However the types which do not subtract the amount of dare, rather compare it with some "example" weight will not be affected much.

Some scales are more sophisticated and completely ignore the initial dare by some methods. They, also, would not exhibit much difference in any gravity or pressure change.

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The word is "tare", not "dare". –  Keenan Pepper Nov 17 '10 at 23:49
    
Sorry, native language contradiction =) –  Cem Nov 18 '10 at 17:32
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