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Re: [Phys-l] definitions ... purely operational, or not



At 13:43 +0800 11/09/2010, carmelo@pacific.net.sg wrote:

Weight as the reading on the bathroom scale may not be appealing when it is applied on an object partially submerged. The object becomes "weightless" because of buoyancy.

If you prefer, the weight can still be Mg. (In this case, g is the centripetal acceleration due to the rotation of the Earth.) There is no freefall acceleration! Perhaps, Bartlett could consider this as apparent weight. (If I am the referee for this paper, that will be my
recommendation!)

Buoyant forces are a bit tricky, and the recent thread here has clearly shown. I think I would look at this from the POV that gravity is still acting on the object, as mg, but the support force that comes from the pressure differences is distributed over the surface of the object. That won't be measured by a bathroom scale in contact with the object, but it will be reflected by a lager reading on the bathroom scale that is supporting the water tank in which the object is immersed. So, I guess that I would not call the object in buoyant equilibrium weightless (but I'm still thinking about it).

Looking at a slightly different situation, consider a person in the gondola of a balloon that is at rest above the surface of the earth. The whole system is being supported by a buoyant force, but the person in the gondola will not feel weightless, and if he is standing on a bathroom scale on the floor of the gondola, it will read a normal weight for the conditions. It's not exactly analogous to the water bouyant effect, but its similar.

Hugh
--

Hugh Haskell
mailto:hugh@ieer.org
mailto:haskellh@verizon.net

It isn't easy being green.

--Kermit Lagrenouille