Weight in a Vacuum
Date: Saturday, June 01, 2002
What happens to the weight of an object in vacuum: does
it decrease, increase or remain the same?
The result depends on the instrument used to do the "weighing," If you use a
scale that has as its measuring member a spring or torsion bar whose
distortion is proportional to weight imposed by the load, the result in a
vacuum would be lower than that obtained in air because air exerts a slight
buoyant force on the object.
If you were to use a balance to do the job, the results would be closer to
the truth because a balance compares the mass of the object "weighed" to the
mass of reference masses that are part of the balance itself. Of course,
buoyancy factors could also enter into this situation as well because air
buoyancy of the object might not be the same as that of the reference
In any case, weighing objects with a balance, rather than a scale, will give
more accurate results because the effects of gravity are compensated for
with the balance and not with the scale. A balance would provide an accurate
measure of your weight (and the same result) whether the determination was
made on earth or on the moon where the acceleration due to gravity is less
than here on earth. Not true with a scale.
Let us first try to clarify what we mean by vacuum and weight, and then
recognize that they are unrelated concepts.
Vacuum, a space free of any material substance, is a concept that commonly
pertains to macroscopic volumes (excluding small volumes at atomic levels. )
One can achieve very high vacuum levels (i.e., very low pressures) in a
small chamber by continuously pumping out to remove suspended molecules in
that volume. Vacuum level achievable in the labs are better than those
in the extra-terrestrial space, yet none is absolute vacuum.
Weight of an object is the force exerted on it by gravity. This force is
the same whether or not the object is in vacuum: gravitational pull is
only affected by the distance between the object and the center of gravity
of the other pulling body (the center of the earth, for instance) and the
masses of the two.
Thus the weight of an object is unaffected if is placed in air or water. A
piece of wood would appear lighter in water but that is because the
buoyancy force counteracts the weight. Weight of the object is
unaffected. Weight, however, is affected if one moves an object from a
low elevation location on earth to a high elevation point (e.g., to a high
mountain) because the distance between it and the earth's center of
gravity is increased. The weight of the object is smaller. Its mass is of
course unchanged because you have removed or added substance to the object.
I hope this clarifies the issue.
Ali Khounsary, Ph.D.
Precision weighings require a correction when the object being weighed is
light, has a large volume, and is made under atmospheric pressure. The
object being weighed displaces a certain amount of air. This creates a
buoyancy force that is equal to the weight of the volume of air displaced. It
is the same effect that occurs if you make a weighing when the object is
weighed under water, only smaller because the density of air is less than
the density of water. If the analytical balance is a double beam tray
balance it is actually the DIFFERENCE in the volume of the object and the
volume of the weights in the other pan. On an electronic balance the buoyancy
is still present but there are no counter weights. The correction is of the
order of a couple of milligrams, but in precision weighing this is not
negligible. There are various designs and techniques to cancel this
correction by keeping the volume of the sample constant and doing a weight
determination by difference, but these techniques are not commonly applied
except in high precision determinations where high accuracy is required.
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