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