Little physics problem

[cyrille de brébisson]

Hello,

In my other "life", I do astronomy... And at the club, last night, someone came up with a good "thought experiment" that might interest some of you (clouds where everywhere, so we could not observe).

"If an object "just floats" in water at Sea level. What will happen at 3000m altitude"

Of course, the question is under defined, which gives rise to lots of ways to answer it. Which is what makes it a "good" question!

Cheers,
Cyrille

P.S.: For those of you NOT in north America, there is a lunar eclipse on the 27th Way worth looking at! (https://www.timeanddate.com/eclipse/map/2018-july-27)

[ijabbott]

(07-26-2018 06:41 AM)cyrille de brébisson Wrote:  "If an object "just floats" in water at Sea level. What will happen at 3000m altitude"

If it's in the Alps, it will be just above the permanent snow line, so the water will probably freeze. It depends what physical state the water was in before the object was placed there.

[Leviset]

Just sign up to Brilliant.org and you can have daily questions like this one - just choose beginner/intermediate/advanced!

[Paul Dale]

The object falls 3km.

[pier4r]

Let's pick a baloon filled with air at the sea level.

it should float on water level, as it is heavy enough to "fall" through the air.
At 3000 m it should pop, or at least expand, as the air pressure around it is lower.

(07-26-2018 07:53 AM)Leviset Wrote:  Just sign up to Brilliant.org and you can have daily questions like this one - just choose beginner/intermediate/advanced!

Yes brilliant is a partially nice site, but stimulating a community is also good (I would be fine in bringing here questions from sites with problems).

I said before "partially" as brilliant , as it is designed like many modern social network, buring older questions/entries (older = some days) under the newest and making the search of older question really hard. Therefore the possibility of enjoying a library of questions is lost. Instead on a forum like this finding old challenges or even answering them after a while is pretty neat.

Careecup is also filled with problems (mostly meh), but then again the discussion of them is suboptimal.

[ijabbott]

If the water remains in the liquid state, its density depends on temperature and pressure (and also mineral content, but let's assume that remains constant). So it depends on how the change in density (w.r.t. temperature and pressure) of the object compares to the change in density of the surrounding water under the same conditions.

[Jlouis]

(07-26-2018 08:04 AM)Paul Dale Wrote:  The object falls 3km.

Oh Paul, you were faster than me!

[Joe Horn]

Doesn't the answer depend on whether the object is airtight (like a glass sphere or submarine) or open to the air (like normal boats)?

Here is my guess: (1) Unless I'm mistaken, an open-air object's buoyancy is determined solely by its mass and the mass of the water it displaces (thank you Archimedes). The surrounding air pressure has nothing to do with it. Therefore lowering the air pressure by bringing the object to a higher altitude would not change its buoyancy.

(2) On the other hand, a totally enclosed object like a glass sphere full of air would keep the same mass but would also physically EXPAND (slightly) when raised to a higher altitude (the way balloons expand as they rise in the atmosphere), thus requiring it to displace (slightly) more water to "just float". The effect would therefore be that the object would float (slightly) higher in the water at 3km altitude than at sea level.

Disclaimer: Stop laughing, I'm no physicist, I'm just guessing here, because it's fun to perform thought experiments like this.

[KeithB]

I am wondering if the "diver in a plastic bottle" applies here. These are the toys that float and sink depending on how hard you squeeze the bottle. It compresses the air bubble in the diver which changes its density and therefore buoyancy.

[Thomas Okken]

(07-26-2018 06:38 PM)Joe Horn Wrote:  Doesn't the answer depend on whether the object is airtight (like a glass sphere or submarine) or open to the air (like normal boats)?

It can get even more complicated than that: scuba divers maintain their depth using a Buoyancy Control Device, BCD for short, which is an inflatable life-jacket like device worn on the torso. They can increase their buoyancy by letting air into the BCD from their air tank, and reduce their buoyancy by letting air out, releasing it into the water.

So, the BCD is like the swim bladder that fish use to control their buoyancy.

The tricky bit is that the BCD provides an unstable equilibrium: if you go up, whether by swimming or by being pushed up by a current, the BCD will expand, because the air inside it is more compressible than the water, and so you become more buoyant, making you tend to rise even further. And when you go down, the air in the BCD gets compressed more than the water, making you less buoyant, and thus tending to sink even lower.

This is similar to how weather balloons can reach such high altitudes: they are barely buoyant when they are launched, but the higher they go, the more they expand, because of the lower pressure.

Both systems would reach a stable equilibrium altitude or depth, if they were rigid. Being airtight is not a sufficient condition for reaching a stable equilibrium.

[Vtile]

Fluids: air and water. Water is liquid so its delta volume vs pressure is a lot smaller than what the pressure vs gas are. When we go up the pressure will drop. If the object is rigid aka not airballoon then its volume weight of air (and also mass because of std g is ~9.815 only at sealevel, but the effect is the same to all objects in this) is more than in sealevel. It will probably float in water slighty less.

..without calculations or analysis at the middle of the night.