Re: Fluids Physlet

[Brian Whatcott <betwys1@SBCGLOBAL.NET>]

It seems to me then that I see a rare note of unanimity here.
Apart from the very, very small depression with pressure
effect on a wood block if porous, the dominant effect
 is to lower the block, when the oil level falls below the
 block's top.

Brian W

At 09:25 PM 8/18/2003 -0400, you wrote:
> Addendum:
>
> In 2) below, c is the total height of the wood block.
> Sorry.
>
> Bob Sciamanda (W3NLV)
> Physics, Edinboro Univ of PA (em)
> trebor@velocity.net
> http://www.velocity.net/~trebor
> ----- Original Message -----
> From: "Bob Sciamanda" <trebor@VELOCITY.NET>
> To: <PHYS-L@lists.nau.edu>
> Sent: Monday, August 18, 2003 9:13 PM
> Subject: Re: Fluids Physlet
>
>
> | I agree with everything you say, Stuart.  I don't think any (A,B or C) is
> | physical.
> |
> | 1.)  When the top of the wood is covered in oil then the fraction of wood
> | floating is:
> | r = (wood - water)/(oil - water), as in my previous post.  This is
> independent
> | of the oil depth - disqualifying B and C.
> |
> | 2) But when the wood top is not covered with oil:
> | r = (water - wood + oil*x/c) /(water), where x is the oil depth.
> |
> | 3) When x=0, this is  r = (water - wood)/ water - the usual Archimedes
> problem.
> | This is clearly smaller than the original r. in  (1)
> | So you are right - the block doesn't move until the oil level goes
> beneath the
> | wood block's top - then it sinks lower. This disqualifies A.
> |
> | Bob Sciamanda (W3NLV)
> | Physics, Edinboro Univ of PA (em)
> | trebor@velocity.net
> | http://www.velocity.net/~trebor
> | | ----- Original Message -----
> | | =46rom: "Stuart Leinoff" <Leinoffs@ACC.SUNYACC.EDU>
> | | To: <PHYS-L@lists.nau.edu>
> | | Sent: Monday, August 18, 2003 3:35 PM
> | | Subject: Fluids Physlet
> | |
> | |
> | | > Greetings,
> | | >
> | | > Ok, I am stumped by the Physlet on fluids found at (among other pla=
> | | ce=3D
> | | > s):
> | | >
> | | > http://webphysics.davidson.edu/physletprob/ch8_problems/ch8_11_flui=
> | | ds=3D
> | | > /fluids_2.html=3D20
> | | >
> | | > The alleged answer is that "B" is the only "physical" animation, bu=
> | | t =3D
> | | > I do not see why the block of wood will float higher in this animat=
> | | io=3D
> | | > n as oil is drawn away but before the block of wood breaks the surf=
> | | ac=3D
> | | > e of the oil.
> | | >
> | | > By Archimedes Principle and the equilibrium principle, the weight o=
> | | f =3D
> | | > the block (constant) should equal the weight of the fluids displace=
> | | d;=3D
> | | >  which would be the sum of the (oil displaced)'s weight and the (wa=
> | | te=3D
> | | > r displaced)'s weight.  Why would the ratio of these two weights ch=
> | | an=3D
> | | > ge when the block is less deep under the oil?
> | | >
> | | > If we calculate the buoyancy force based on the difference between =
> | | th=3D
> | | > e water's pressure pushing up on the bottom surface of the block an=
> | | d =3D
> | | > the oil's pressure pushing down on the top surface, again I do not =
> | | se=3D
> | | > e why this "difference" would be different before the oil no longer=
> | |  c=3D
> | | > overs the top surface of the block.
> | | >
> | | > Animation "A" does not seem physical to me because once the block i=
> | | s =3D
> | | > no longer under the oil, it should sink deeper into the water.
> | | >
> | | > I would appreciate someone else's insight into this.
> | | >
> | | > Thanks,
> | | >
> | | >
> | | > Stuart Leinoff
> | | > Professor of Physics
> | | > Science Division Chair
> | | > ACC


Brian Whatcott    Altus OK    Eureka!