Re: [Phys-l] balloon floating in air in car

[Brian Whatcott <betwys1@sbcglobal.net>]

This story of David's works perfectly well - but it is in my view not 
central to the mechanism.
Suppose that a buoyancy force tends to move an object in the effective 
direction of "up".    And suppose that an accelerating frame changes the 
effective direction of "up".
Then a buoyant object will adjust its preferred direction if 
unconstrained, in THAT direction!   Any motion of the medium is a 
consequence of the buoyant object's movement.    That works quite nicely 
for the helium in the accelerating car case, but what of the 
elevator/lift case?

"Up" stays "Up" until it becomes "Down" no doubt. Does the effective 
weight of the buoyant object change? Yes.
Does the weight of the displaced medium change? Yes   Does the flotation 
level change? No. Not until "Up" becomes "Down" no doubt. What about the 
zero g case?
No "Up" so no buoyant force.... but there are other forces...of which 
the one that seeks to minimize surface area of a liquid comes into play 
- that is, if there is an object that penetrates that surface, so that 
it is likely to be ejected, otherwise it may stay ensconced, as 
contributors have noted.

About density gradients and compression in air - computational 
aerodynamics folks find it is perfectly practical to treat air rushing 
over wings as incompressible until quite high sub-sonic speeds.  
Now wait a darn' minute! Don't we all know a wing gets two thirds of its 
lift from the rarifaction over the upper surface, and a third from the 
compression over the lower wing surface?
             Yes....and no.

There! Such an amalgam of unsupported speculation. It is of course 
true   :-)

Brian W

David Willey wrote:
> The balloon moves forward as the denser air moves to the back of the 
> vehicle,
> cheers,
> David
>
> Richard A. Lindgren wrote:
>   
> > Hello,
> > Another variation. How about a helium filled balloon floating  gently up 
> > against the roof in the back seat of your car. What happens when the car 
> > accelerates forward?
> > Richard at UVa
> >
> > On Tue, 21 Apr 2009 11:25:39 -0400
> >   "LaMontagne, Bob" <RLAMONT@providence.edu> wrote:
> >   
> >     
> > > Obviously the ball goes up with the elevator.
> > >
> > > Bob at PC
> > > ( mg = rho g V,  g increases on both sides of the equation, so V doesn't 
> > > change?)
> > >
> > > -----Original Message-----
> > > From: phys-l-bounces@carnot.physics.buffalo.edu 
> > > [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Anthony 
> > > Lapinski
> > > Sent: Tuesday, April 21, 2009 8:38 AM
> > > To: phys-l@carnot.physics.buffalo.edu; tap-l@lists.ncsu.edu
> > > Subject: [Phys-l] ball floating in elevator
> > >
> > > A ball is floating in a beaker of water in an elevator. What happens to
> > > the ball when the elevator accelerates upward? Good question for your
> > > "bright" students!
> > >
> > > Think about this before checking out the video below.
> > >
> > > http://www.youtube.com/watch?v=OXuYWCWIaJI
> > >
> > > I guess you could demonstrate an elevator accelerating downward -- free
> > > fall -- by dropping a (plastic) beaker above a garbage pail to see what
> > > the ball does while falling. Might be hard to observe this fast motion, so
> > > a video camera would be useful.
> > >