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

[Jeffrey Schnick <JSchnick@Anselm.Edu>]

> -----Original Message-----
> From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of John Denker
> Sent: Thursday, January 30, 2014 2:37 PM
> To: Phys-L@Phys-L.org
> Subject: [Phys-L] connecting buoyancy to force +- density gradient
>
> On 01/30/2014 08:32 AM, Jeffrey Schnick wrote:
>
> > we cause more downward momentum to flow into it by bringing a steel
> > ball (part of the surroundings) into contact with the water.
>
> Slight nitpick:  The beauty of the momentum-flow approach is that it doesn't
> directly depend on the ball being in contact with the water.  The key idea is
> momentum flow /across the boundary/.  So one of the crucial steps is
> bringing the steel ball inside the boundary of the RHS subsystem.

Interesting.  In my description  of the movie corresponding to your solution,

[In the movie that corresponds to your solution, on the left we start with a beaker of water with downward momentum (represented by little down arrows) flowing through it, in via gravity and out via contact of the bottom of the beaker with the pan of the pan balance.  We add a table-tennis ball to the system and the flow increases a tiny amount.  On the right, we start with an identical beaker of water (with the same original momentum flow) and a steel ball hanging by a string with downward momentum flowing through it, in via gravity out via the string.  We lower the ball into the water and a significant portion of the momentum flow through the ball is redirected so that if flows into the water via the contact of the ball with the water.  Significantly more downward momentum is flowing into the beaker-plus-water on the right, so that side goes down.],

the ball started out inside the boundary, perhaps below the rim of the beaker but above the water level.  In lowering the ball into the water, a negligible amount of string enters the region inside the boundary.  More importantly, the ball makes contact with the water at which point some of the downward momentum flowing via gravity into the ball part of the system, momentum which was all flowing out via the string, now starts flowing out down through the water and then through the bottom of the beaker.  That redirection of momentum does not occur until the ball makes contact with the water.  The ball/water contact is what opens up the new avenue for momentum to flow out of the ball and, to me, is thus crucial.