# physics queston: pressure at bottom of sea
**URL:** https://boards.straightdope.com/t/physics-queston-pressure-at-bottom-of-sea/541144
**Category:** Factual Questions
**Created:** [May 29, 2010, 3:57am UTC](https://boards.straightdope.com/t/physics-queston-pressure-at-bottom-of-sea/541144 "2010-05-29T03:57:47Z")
**Posts on this page:** 20
**Page:** 2
### Author:  [@Oukile](https://boards.straightdope.com/u/Oukile)
#### Post date: [May 29, 2010, 5:43pm UTC](https://boards.straightdope.com/t/physics-queston-pressure-at-bottom-of-sea/541144/21 "2010-05-29T17:43:03Z")
In fact, (assuming non-sealed scale), the part on which you step (just above the spring), would be pushed upward by Archimede force. So the scale would read a small negative value.
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### Author:  [@Boyo\_Jim](https://boards.straightdope.com/u/Boyo_Jim)
#### Post date: [May 29, 2010, 5:44pm UTC](https://boards.straightdope.com/t/physics-queston-pressure-at-bottom-of-sea/541144/22 "2010-05-29T17:44:15Z")
> [@ivan\_astikov](#):
>
> So, what would the reading show if you placed something equal to your weight that wasn’t buoyant on it?
The less buoyant you are, the more weight you will register on the scale. I don’t know that it is possible for someone to be totally non-buoyant.
Actually, this raises another question. Positive buoyancy floats, negative buoyancy sinks. Is there something like an absolute zero of buoyancy?
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### Author:  [@Boyo\_Jim](https://boards.straightdope.com/u/Boyo_Jim)
#### Post date: [May 29, 2010, 5:46pm UTC](https://boards.straightdope.com/t/physics-queston-pressure-at-bottom-of-sea/541144/23 "2010-05-29T17:46:11Z")
> [@Oukile](#):
>
> In fact, (assuming non-sealed scale), the part on which you step (just above the spring), would be pushed upward by Archimede force. So the scale would read a small negative value.
WTF is Archimede force?
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### Author:  [@Pasta](https://boards.straightdope.com/u/Pasta)
#### Post date: [May 29, 2010, 5:54pm UTC](https://boards.straightdope.com/t/physics-queston-pressure-at-bottom-of-sea/541144/24 "2010-05-29T17:54:46Z")
> [@Boyo\_Jim](#):
>
> It wouldn’t work fine under water, in the sense of accurately measuring your weight. You are buoyant – if you stand on it under water, it won’t give a true reading.
It will accurately report the net downward force, which it what it is designed to do. Yes, buoyancy is important in translating the reading into information about your own mass, but that’s a separate issue. (After all, you don’t normally worry about the the 0.5 lbs of buoyancy you have in air when using the scale on the surface.)
My point, in any case, is simply that the scale won’t suffer a fault like an electronic one would.
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### Author:  [@Pasta](https://boards.straightdope.com/u/Pasta)
#### Post date: [May 29, 2010, 5:58pm UTC](https://boards.straightdope.com/t/physics-queston-pressure-at-bottom-of-sea/541144/25 "2010-05-29T17:58:02Z")
> [@kombatminipig](#):
>
> Scale it up a bit and make it square, and you have a bathroom scale.
Missing the critical (answer-changing) piece: scale it up a bit and make it square and the _puncture the cavity beneath the pressure plate to allow the pressure to equalize_ and you have a bathroom scale.
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### Author:  [@Pasta](https://boards.straightdope.com/u/Pasta)
#### Post date: [May 29, 2010, 6:02pm UTC](https://boards.straightdope.com/t/physics-queston-pressure-at-bottom-of-sea/541144/26 "2010-05-29T18:02:47Z")
> [@](#):
>
> WTF is Archimede force?
He is referring to the fact that if the scale was “zeroed” in air, the spring would be perfectly balanced against the weight of the plate and associated gear. Underwater, though, the plate _et al._ will displace some water (_i.e._, be buoyant), so the spring would be pushing back too strongly by several tenths of pounds.
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### Author:  [@Buck\_Godot](https://boards.straightdope.com/u/Buck_Godot)
#### Post date: [May 29, 2010, 6:03pm UTC](https://boards.straightdope.com/t/physics-queston-pressure-at-bottom-of-sea/541144/27 "2010-05-29T18:03:11Z")
wouldn’t it depend on the type of fish to took the scales from?🙂
-runs away
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### Author:  [@Napier](https://boards.straightdope.com/u/Napier)
#### Post date: [May 29, 2010, 10:22pm UTC](https://boards.straightdope.com/t/physics-queston-pressure-at-bottom-of-sea/541144/28 "2010-05-29T22:22:20Z")
> [@UTejas](#):
>
> **Xema** and **pancakes3** are correct. Everything else is nonsense.
>
> The scale weighs the downward component of weight only. When you step on it on the bathroom floor, it doesn’t register the normal force upwards from the floor and tell you that you weigh 0 lbs. In this case forces are balanced, as they should be since neither you nor the scale are accelerating, and the scale doesn’t register 0 lbs.
>
> It is the same with a spring scale. When you hang a weight on a spring scale it doesn’t register 0 even though the spring force is balancing the gravitational force.
>
> It will measure the 2000 psi X 100 square inches.
Wow.
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### Author:  [@Jeff\_Lichtman](https://boards.straightdope.com/u/Jeff_Lichtman)
#### Post date: [May 29, 2010, 11:17pm UTC](https://boards.straightdope.com/t/physics-queston-pressure-at-bottom-of-sea/541144/29 "2010-05-29T23:17:47Z")
This is easily tested - water is dense enough that you don’t need to put the scale at the bottom of the ocean. Only a few feet of water will do. Let’s say you submerge a scale with a one square foot platform in three feet of water. The column of water above the scale will weight about 187 pounds, yet the scale will read zero under these conditions. Try it if you don’t believe me.
Actually, a scale that reads zero in air might read a little less than zero underwater. The platform itself has weight, and the scale must be calibrated for it. Buoyancy could throw off the calibration. Of course, this wouldn’t be true for a balance scale.
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### Author:  [@Diver](https://boards.straightdope.com/u/Diver)
#### Post date: [June 1, 2010, 10:08pm UTC](https://boards.straightdope.com/t/physics-queston-pressure-at-bottom-of-sea/541144/30 "2010-06-01T22:08:23Z")
> [@Boyo\_Jim](#):
>
> The less buoyant you are, the more weight you will register on the scale. I don’t know that it is possible for someone to be totally non-buoyant.
>
> Actually, this raises another question. Positive buoyancy floats, negative buoyancy sinks. Is there something like an absolute zero of buoyancy?
Yes.
Scuba divers wear a BCD (buoyancy control device) to achieve neutral buoyancy.
Small amounts of of air can be added from the tank to increase buoyancy or vented from the BCD to reduce it.
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### Author:  [@Boyo\_Jim](https://boards.straightdope.com/u/Boyo_Jim)
#### Post date: [June 1, 2010, 10:18pm UTC](https://boards.straightdope.com/t/physics-queston-pressure-at-bottom-of-sea/541144/31 "2010-06-01T22:18:09Z")
> [@Diver](#):
>
> Yes.
> Scuba divers wear a BCD (buoyancy control device) to achieve neutral buoyancy.
>
> Small amounts of of air can be added from the tank to increase buoyancy or vented from the BCD to reduce it.
No, that’s not what I meant. I know there is positive and negative buoyancy based on whether the object rises or sinks in a given water column. I don’t know what the unit of measurement is for this, but presumably there must be negative numbers involved in some cases.
My question is whether there is an absolute zero of buoyancy, like in temperature, which is -273 F degrees IIRC.
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### Author:  [@Lemur866](https://boards.straightdope.com/u/Lemur866)
#### Post date: [June 1, 2010, 10:31pm UTC](https://boards.straightdope.com/t/physics-queston-pressure-at-bottom-of-sea/541144/32 "2010-06-01T22:31:32Z")
> [@Boyo\_Jim](#):
>
> No, that’s not what I meant. I know there is positive and negative buoyancy based on whether the object rises or sinks in a given water column. I don’t know what the unit of measurement is for this, but presumably there must be negative numbers involved in some cases.
>
> My question is whether there is an absolute zero of buoyancy, like in temperature, which is -273 F degrees IIRC.
No, because bouyancy depends on the density of the medium the object is displacing. If an object has lower density than the medium, it will rise. If it has higher density it will sink. If it has exactly equal density it will be stationary.
The absolute maxiumum bouyancy an object could have is if it had zero mass–if it was a bubble of vacuum. The problem is that the medium would collapse the bubble, so you’d have to have a rigid container to hold the vacuum, and the rigid container would have mass.
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### Author:  [@Boyo\_Jim](https://boards.straightdope.com/u/Boyo_Jim)
#### Post date: [June 1, 2010, 10:39pm UTC](https://boards.straightdope.com/t/physics-queston-pressure-at-bottom-of-sea/541144/33 "2010-06-01T22:39:25Z")
So a black hole would theoretically be the least buoyant object in the universe? Of course, it would suck in all the water around it, so it can’t really be said to matter a whole lot.
What is the least buoyant material that can continue to exist in water, and water can continue to exist around it? Lead? Plutonium? My ignorance of the periodic table is breathtaking.
Um, would this involve the periodic table?
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### Author:  [@yoyodyne](https://boards.straightdope.com/u/yoyodyne)
#### Post date: [June 2, 2010, 12:10am UTC](https://boards.straightdope.com/t/physics-queston-pressure-at-bottom-of-sea/541144/34 "2010-06-02T00:10:26Z")
> [@Boyo\_Jim](#):
>
> What is the least buoyant material that can continue to exist in water, and water can continue to exist around it? Lead? Plutonium? My ignorance of the periodic table is breathtaking.
>
> Um, would this involve the periodic table?
[Osmium](http://en.wikipedia.org/wiki/Osmium), if you’re limiting it to material found on earth 🙂
Density isn’t necessarily on a periodic table.
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### Author:  [@Jeff\_Lichtman](https://boards.straightdope.com/u/Jeff_Lichtman)
#### Post date: [June 2, 2010, 1:58am UTC](https://boards.straightdope.com/t/physics-queston-pressure-at-bottom-of-sea/541144/35 "2010-06-02T01:58:43Z")
> [@yoyodyne](#):
>
> [Osmium](http://en.wikipedia.org/wiki/Osmium), if you’re limiting it to material found on earth 🙂
Osmium is the densest element, but is it the densest substance? Are there compounds with a density greater than osmium’s?
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### Author:  [@kombatminipig](https://boards.straightdope.com/u/kombatminipig)
#### Post date: [June 2, 2010, 5:12am UTC](https://boards.straightdope.com/t/physics-queston-pressure-at-bottom-of-sea/541144/36 "2010-06-02T05:12:22Z")
> [@Pasta](#):
>
> Missing the critical (answer-changing) piece: scale it up a bit and make it square and the _puncture the cavity beneath the pressure plate to allow the pressure to equalize_ and you have a bathroom scale.
True, but we must be discussing an ideal scale in this case, or the entire question is moot.
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### Author:  [@kombatminipig](https://boards.straightdope.com/u/kombatminipig)
#### Post date: [June 2, 2010, 5:13am UTC](https://boards.straightdope.com/t/physics-queston-pressure-at-bottom-of-sea/541144/37 "2010-06-02T05:13:35Z")
> [@Diver](#):
>
> Yes.
> Scuba divers wear a BCD (buoyancy control device) to achieve neutral buoyancy.
>
> Small amounts of of air can be added from the tank to increase buoyancy or vented from the BCD to reduce it.
Beer comes in bottles, soldiers come in tanks, air comes in cylinders 😛
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### Author:  [@Pasta](https://boards.straightdope.com/u/Pasta)
#### Post date: [June 2, 2010, 5:07pm UTC](https://boards.straightdope.com/t/physics-queston-pressure-at-bottom-of-sea/541144/38 "2010-06-02T17:07:38Z")
> [@kombatminipig](#):
>
> True, but we must be discussing an ideal scale in this case, or the entire question is moot.
I guess we’d need the OP to return to determine that definitively. I see the question with a regular scale as being far from moot. The idea of pressure acting as a force in all directions is an oft-misunderstood concept, and I could certainly see this being the crux of the original question.
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### Author:  [@Chessic\_Sense](https://boards.straightdope.com/u/Chessic_Sense)
#### Post date: [June 2, 2010, 5:48pm UTC](https://boards.straightdope.com/t/physics-queston-pressure-at-bottom-of-sea/541144/39 "2010-06-02T17:48:04Z")
> [@kombatminipig](#):
>
> Beer comes in bottles, soldiers come in tanks, air comes in cylinders 😛
Elephants come in gallons.
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### Author:  [@Napier](https://boards.straightdope.com/u/Napier)
#### Post date: [June 2, 2010, 10:19pm UTC](https://boards.straightdope.com/t/physics-queston-pressure-at-bottom-of-sea/541144/40 "2010-06-02T22:19:58Z")
I think osmiridium, an alloy of osmium and iridium, is the densest substance on earth. If you claim the nucleus of an atom to be your sample, it’s far denser than that, but you can’t isolate it from its cloud of electrons, and is probably an example of denser that only satisfies the word of the OP and not its spirit. Earthly substances that occur in other places where pressure is higher could be more dense, and various kinds of condensed matter can be far more dense.
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