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Name: Unknown
Status: educator
Age: 30s
Location: N/A
Country: N/A
Date: Saturday, June 01, 2002


Question:
What happens to the weight of an object in vacuum: does it decrease, increase or remain the same?


Replies:
Educator,

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 masses.

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.

Regards,
ProfHoff 434


Hello,

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.

Vince Calder



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