Re: [Phys-l] buoyancy on a submerged pole

[John Mallinckrodt <ajm@csupomona.edu>]

Chuck Britton wrote:

> I like J. Clements use of PSEUDO-force for the Upward Force exerted on a submerged object.

Mmmmm, well.  Maybe I didn't hear that from John C., but I hope he would agree that the buoyant force is not in the same category as those that we generally refer to as "pseudo-forces."  

> Pseudo-forces are convenient when one wants to sweep a lot of detail under the rug.

Generally the term "pseudo-force" refers to "inertial forces" that arise as a result of being in an accelerating frame.  Today we understand gravity to be such a force.

> I wish we would agree refer to 'Contact Force', 'Normal Force' etc as 
> pseudo-forces.

I wouldn't recommend that.  Sounds like a prescription for misunderstanding.  An object that is NOT submerged in (i.e. completely surrounded by) a fluid in hydrostatic equilibrium may or may not experience "buoyancy," but it will not generally be the case that it is subject to ANY force, pseudo or otherwise, that is equal to the weight of whatever fluid it displaces.

> MY fbd incudes the upward pseudoforce due to the displaced fluid 
> which measurably affects the 'contact' pseudoforce that we are all 
> pretending to understand so thoroughly.

Hmm.  Therein, I suspect, lies the problem!  You would have the box subject to the following three forces:

1. Buoyant force equal to the weight of the displaced water (UPWARD)

2. Gravitational force smaller in magnitude than the buoyant force because the box is less dense than water (DOWNWARD)

3. A contact force that must balance the other two and therefore be (DOWNWARD)

But because 1 is wrong, 3 is wrong.

John Mallinckrodt
Cal Poly Pomona