Re: [Phys-L] connecting buoyancy to force +- density gradient

[John Denker <jsd@av8n.com>]

On 01/31/2014 12:48 PM, Jeffrey Schnick wrote:
> the ball started out inside the boundary, perhaps below the rim of 
> the beaker but above the water level.

OK.

> the ball makes contact with the water at which point some of the
> downward momentum .... now starts flowing out down through the water
> and then through the bottom of the beaker.

That works.

The central idea is the process-of-elimination argument:
Whatever momentum doesn't flow out through the string has 
to flow out through the scale-arm, because those are the 
only two ways that momentum can flow out.

   (It flows in via gravity.)

The details don't matter, and the momentum-flow approach
makes it clear that the details don't matter.  The momentum-
flow approach focuses attention on the two bottlenecks, the 
two places where momentum can flow out.  It automagically
focuses attention on what's important.

The same logic applies with even greater simplicity and power
to the left side:  The ping-pong ball could be floating, or
anchored to the bottom, or resting on the balance-pan beside 
the beaker, high and dry.  Those are all the same, because 
none of them provide any way for momentum to cross the boundary 
of the left subsystem.