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Name: Phyllis
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Question:
Which weighs more: a 100 pound rock on dry land or a 100 pound rock totally submerged in water?



Replies:
Please clarify, Phyllis-

If they are both 100 lb, can they both be the same rock? Or can they be different rocks with equal mass (having both been weighed and found to be 100 lb on dry land)? If so, then the rock under water weighs less by the weight of an equal volume of water.

Present consensus meaning of terms: Mass is the true amount of matter, measured by its inertia. (i.e., how much force is required to cause a certain acceleration, in absence of any drag forces.) Weight is the force which gravity exerts on that mass, in the immediate situation, which changes when you enter or leave the water, or go to a different planet.

So, reading your question literally, _and_ assuming that pounds ("Lb") is a measure of weight (rather than "mass-normally-having-that-weight"), perhaps both rocks weigh the same, by your stipulation. Perhaps each was measured to weigh 100 lb in its respective environment, and now you are asking me to compare their weights, while they are presumably still in their same respective environments. (Then the rock under water has same weight, but more mass, and must be a different rock.)

Sorry I am so humorlessly analytical about the words. It has never been clear, in my opinion, whether the common unit "pound" refers first to the mass or the weight-force.

So it is common in enginering situations to distinguish between pounds of mass ("lbm") and pounds of force or weight ("lbf"). In my opinion, this is what is currently required to take the tripping ambiguity out of this type of situation-description, in other words, to be fair to the reader.

Sometimes people wish to use grams and kilograms to express force, too. I do it often. "Kilograms", used alone, explicitly refers to mass. When referring to force: "Kilogram-force" or "kgf" would be a clear way to refer to the force typically generated by 1kg of mass in one earth gravity. (1kgf=9.8Newtons, and 1kg= (uh...) 1kg.)

Jim Swenson



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