Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

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



At 11:21 AM -0700 11/5/10, John Mallinckrodt wrote:
Chuck Britton wrote:

I THINK that my hypothesis can be restated as saying that an object
submerged in a fluid will experience an UPWARD force that is equal to
the weight of the fluid displaced.

This sounds a lot like the misconception many students have that IF an object is moving along a circular path THEN it is subject to a "centripetal force" equal to mv^2/r . That "centripetal force" must, therefore, be added to any other forces that might act on the object.

To avoid making that mistake, it's worth reminding them (and ourselves!) that forces always have local "agents" either in the form of "fields" (e.g., gravitational or electromagnetic) or "via direct contact." If you can't identify the agent, the force does not exist.


You are absolutely correct. I like J. Clements use of PSEUDO-force for the Upward Force exerted on a submerged object. Pseudo-forces are convenient when one wants to sweep a lot of detail under the rug. I wish we would agree refer to 'Contact Force', 'Normal Force' etc as pseudo-forces.

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.