A Physics Question.  What would result if 15 lb. plastic jar with a
solid metal and plastic core were dropped into the Mid-Atlantic Ocean
where depths average 1 to 2 miles.  Would the item sink to the bottom?
 Or reach equilibrium at some interim point?

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Clarification of Question by lamarchant-ga on 28 Apr 2005 20:33 PDT

Clarification in case any doubt exists due to wording of question. 
Total weight of item is 15 lbs.

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Request for Question Clarification by efn-ga on 28 Apr 2005 22:50 PDT

What is the volume of the jar?

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Clarification of Question by lamarchant-ga on 29 Apr 2005 10:09 PDT

To answer question as to volume.  It has been many years since my one
course in Physics 101.  Best I can say is plastic cylinder [i.e.
container] is aprx 14 " high by 7" in dia. at top, tapering to aprx 5"
at base.

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Clarification of Question by lamarchant-ga on 29 Apr 2005 10:42 PDT

Clarification for ticbol-ga:  Your analysis much appreciated. 
Capsules [there are 3] already put in place on a line running from
Bermuda to St. Thomas.  Idea was to avoid thermal area lying further
east. Hope this has been achieved.  Expectation for recovery depends
on when ocean bottom mining goes into effect.  200 years?  500 years?
1000?  or never!

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Request for Question Clarification by hedgie-ga on 29 Apr 2005 21:21 PDT

lamarchant-ga 


Comment by bavetta-ga provided the correct analysis.
Assumption is that metallic container will not be crushed
- since we do not know the details - it is a reasonable assumption.

You may expire the question and your answer is free -
or clarify what is still not answered.

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Clarification of Question by lamarchant-ga on 30 Apr 2005 10:18 PDT

Many thanks to all who have replied to date to this time capsule
question!  I am not sure how to answer regarding  "how deep is the
capsule"  Again it is a plastic jar 14" x 7" [at top] tapering to 5"
[at base].  The core is a brass plumbers pipe [with which we all are
familiar] about 8" in length and about 1 3/4" in diameter which
contains the data, fastened at each end with a screw on closure.  A
rather formidible unit!  All of this surrounded by a liquid plastic
hardened to appear rather dense. A rather solid looking unit, at least
at ground level.  All of this from memory, as the capsule(s) are no
longer here. Will let this question ride over the weekend to see what
additional information develops, but it may be a puzzle with no
certain conclusion!

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Request for Question Clarification by efn-ga on 30 Apr 2005 11:49 PDT

I agree with most of the commenters and my colleague hedgie that the
time capsule will sink to the bottom.  lamarchant, are you still in
doubt about this conclusion?  If so, perhaps you could explain your
doubts.

If the thing started to sink, it will continue to sink until something
stops its sinking.

Sinking. Vertical displacement. Downwards.
Vertical forces, either direct or components of non-vertical foces,
act on the capsule.
The thing sinks because the downward forces are greater than the
upward foces. There is a resultant downward force that sinks the
capsule.

What is the main downward force acting on the sinking capsule?
The 15-lb. total weight of the capsule, due to acceleration due to gravity.

What are the main upward forces acting on the sinking capsule?
The bouyant force, and friction.

The fact that the capsule keeps on sinking after being thrown
overboard, it means the bouyant force is less than the weight of the
capsule. If the bouyant force is greater than 15 lbs, then the capsule
will rise to the surface and float after momentarily sinking a bit due
to the accleration caused by the fall/overthrow.

As the capsule sinks deeper, what will stop the sinking? The
hydrostatic pressure? Because the hydrostatic pressure increases as
the depth increases, then there would be a point when the bouyant
force will increase to stop the sinking?
No. Because bouyant force is due to the weight of the ocean water
only---not to hydrostatic pressure. The hydrostatic pressure will
neutralize itself as far as movement of the capsule is concerned. This
pressure attacks the capsule in all directions.

Friction will stop the sinking? Like the capsule would be
gripped/squeezed by the hydrostatic pressure?
No. The viscosity of ocean water is not enough to offer that much friction.

Therefore, the capsule will hit bottom eventually.

--------
If an aquatic animal, a human, a submarine, a fishnet, whatever, snags
it before it reaches the bottom of the ocean, then the capsule will
not reach the bottom---if it is not let go.

Since it is in Mid-Atlantic, the capsule might not reach bottom if it
got caught by convection current due to the heated water at the
Mid-Atlantic Ridge. That is if the capsule's destination is aligned
with the super hot crack, if the current is strong enough to lift the
capsule, if the capsule cannot escape the current eventually.

Which brings out the question, why choose the Mid-Atlantic? The time
capsule won't be found later on if it were sinked away from the
earth-split? What if it enters the earth there? Is the plastic vessel
designed not to melt at any heat/temperature? Boy, it sure is very hot
down there!

Greetings,
I disagree with ticbol-ga.  I believe lamarchant-ga is describing a
plastic jar contain metal (heavier than water) and plastic (much
lighter than the metal and serving as a mass to keep the jar from
compressing), all wighing 15 lb in the air and too heavy to float on
the surface of the water. So it sinks, but at some level the
hydrostatic pressure will equal the weight of the jar, giving it
neutral bouyancy.  It will remain suspended at this level.

Submarines work on this principle, having tanks that can be filled
with water to increase the weight of the vessel, or be emptied of
water by releasing compressed air into the tanks.

Here is a site that describes a variable bouyancy system:

http://www.oceanlab.abdn.ac.uk/news/vbs.shtml

efn-ga's request for clarification (volume of the jar?) is
appropriate, since in the scenario presented, it could be that the jar
would sink to the bottom (1-2 miles) if the jar continued to have
negative bouyancy to this depth, something which can be calculated (
not by me!) if the volume of the jar is known.

And I respectfully disagree with myoarin.

The pressure of the water has nothing to do with buoyancy; as ticbol
points out, pressure is omnidirectional so it pushes down as hard as
it pushes back up. All that matters is the density of the water. When
the density of the water is equal to the density of the capsule,
neutral buoyancy will be achieved (this is equivalent to saying that
the weight of the volume of water displaced by the capsule is equal to
the weight of the capsule). However, since water is pretty much
incompressible, the density at any depth is not going to change enough
to float the capsule.

What does have an effect on water density is the salinity, but again
there isn't enough range to be of significance.

Since the density of ocean water does increase slightly with depth, in
principle you could design a capsule that would just barely sink at
the surface, but come to an equilibruim position at some depth. 
However, I think this would require very fine-tuning of the depth and
compressability of the capsule.

Submarines rise or sink based on how much they displace and how much
they weigh.  They try to achieve a neutral boyancy and go up and down
by using their planes (like the wings on an airplane)  It is true that
as they go deeper the water temerature changes and the water becomes a
little denser. (due to temperature)  This does make the sub more
boyant but not much.  With subs they want to know their weight very
accurately so they know how much water to take on to become
neutral.(so they add and subtract as things come on and go off the
sub, when they are in dock)

The same is true of scuba divers.  They wear weight so they are
neutral boyancy.  As they decend their boyancy changes a little bit
because the wet suit (or dry suit) compresses due to they hydrostatic
pressure.  The amount is not a lot.  You get a larger effect by
breating in and out.  as you scuba dive you can feel yourself rise
slightly as you breath in (your lungs get larger with air) and sink as
you breath out.  You can even control your boyancy somewhat by how
much air you retain in your lungs.(while you breath.  NEVER HOLD YOUR
BREATH WHILE YOU ARE SCUBA DIVING!!!)  You also wear a BC or boyancy
control device that you can put more air in to help control your
neutral boyancy.  You want to be neutrally boyant while scuba diving.

The density of ocean water essentially stops growing with depth at
around 1028 kg/m^3.  The maximum volume of the container is (14
inches) * 7^2 inches * pi = 0.035 m^3.   The weight of the water that
the container is is displacing at great depths is .035 m^3 * 1028 Kg 
= 35.98 N (force up).  15 pounds = 66.72 N (force down) - the
container will sink to the bottom.

The bigger problem is pressure.  Pressure goes like P = P-atmospheric
+ density*gravity*depth.
So, at 2 miles (3218.688 meters) the pressure outside the vessel is
32,559,438.5 Pascals, or 4,722 Pounds per Square Inch. - The thing
might get crushed....

myoarin-ga got mixed-up.

A submartine has a constant volume, so the buoyant force acting on it
is constant. Buoyant force is equal to the weight of liquid that is
displaced by an object. The volume of this displaced liquid is the
volume of the object. So, to repeat, if the volume of the object is
constant, then the buoyant force is constant.

A submarine cannot go against Physics. If it is sinking, it will
continue to sink until some force is applied to slowdown, stop, or
reverse the sinking. The entry or exit of seawater into or from the
submarine ballasts regulate this vertical movement. If water is let
into the ballasts, the weight of the submarine will increase---but the
volume of the submarine remains the same, of course---so the sub will
sink. If water is released, the sub will weigh less, so the sub will
rise.

What about the hovering, the lying still at the same depth?
Physics again. An object at rest will remain at rest until some force
is applied to move the object.
If the sub is sinking, water is let out to raise the sub. Once the
sinking is stopped, or the sub is at rest---after some manipulation or
fine-tuning of the water movement in/from the ballasts---the ballasts
are closed. Now the weight of the sub is equal to the buoyant force.
All vertical forces are neutralized. There is no more resultant
vertical force. With nothing to move the sub either up or down, the
sub hovers.

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Something to think over:

Why do dust falling on the surface of the ocean do sink to the bottom
eventually? Why do debris, of any size and shape, that sinks that are
thrown overboard from ships eventually sink to the bottom? Why do we
not see any suspended or hovering object underwater in
pictures/films/videos showing underwater or regions under the water?

Back to the submarine, could you imagine the unceasing tension on the
crew if "pings" or blips of suspended objets are unceasingly "heard"
or seen on the monitor?

Nemo scould have been warned also by his loving Dad to always watch
out for these suspended objects.
Knowing Nemo, he might even play with some of these hovering objects,
or get some to show his father. "Look, Dad, I've got a TV this time.
Let us watch Finding Nemo."

I am happy to have been mixed up.  I am one those persons who is
basically denser than water (not just above the ears  (-: ), sinking
unless I have my lungs really expanded, but I am still not entirely
convinced (only a fool never chances his mind).

lamarchant-ga, how deep are the capsules?

The volume of the container is (this is just one way to calculate it):
(3.5^2*pi*7)+ (2.5^2*pi*7) = 406.836 in^3
15lbs/406in^3 = .0368677 lbs/in^3
Converted to g/ml = 1.0205 g/ml
So comparing to a chart such as the one on p. 2:
http://sealevel.jpl.nasa.gov/education/activities/ts2ssac3.pdf
you'd have to have very high temp and very low salinity before your
your capsule would sink at all. (Despite most of these silly
responses, buoyancy depends only on densities-- upward force is equal
to the weight of the water displaced by the object).