Wikipedia:Reference desk/Archives/Science/2022 October 12

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October 12

Metal object on the top of styrofoam balls

A metal object on the top of a box of styrofoam balls (the kind used for filling a package) would not necessarily sink to the bottom (unless some shaking is applied). I imagine that that's because the friction between styrofoam balls, that locks them in place. Is this intuition right? And could you put some object on the top of a liquid with lower density in a similar way (without it sinking)? Bumptump (talk) 20:08, 12 October 2022 (UTC)

Water certainly has a high surface tension that allows you to float heavier objects carefully on it. Rmhermen (talk) 00:45, 13 October 2022 (UTC)

But could we say that, for example, a needle floating in water it's the same physical phenomenon as in the styrofoam balls case above? Bumptump (talk) 01:00, 13 October 2022 (UTC)

No, it is a different phenomenon; it has some superficial similarities, but (perhaps obviously!) solid materials differ from liquid materials (and perhaps non-obviously!) this manifests in profound ways when we study the complicated dynamics of such systems.

The behavior of a group of solid spheres in profoundly different from the behavior of liquid molecules - and despite the appeal of using analogy to model liquid molecules as "very very small" solid spheres, that's just not quite right.

We have an article on granular materials, which are an actively studied area of applied condensed matter physics.

Nimur (talk) 03:10, 13 October 2022 (UTC)

Part of the reason why the metal object stays on top of the styrofoam balls is friction. The other part is that the metal object can only move down when the styrofoam balls move up. The geometry may be such that the styrofoam balls must initially rise much faster than the metal object sinks, so we get a local minimum in the potential with the metal object still on top.

As you correctly observe, with a bit of shaking, the metal object will sink. In water, there's always shaking: Brownian motion. If we stop Brownian motion by freezing the water, the metal object can stay on top again. PiusImpavidus (talk) 09:42, 13 October 2022 (UTC)

A B52

Surface tension is sufficient if the object is small enough; things like paper clips and small needles and the like can be floated on water if one is careful to not submerge them under the surface. --Jayron32 11:02, 13 October 2022 (UTC)

This discussion reminds me of many interesting non-scientific evenings. MinorProphet (talk) 21:20, 15 October 2022 (UTC)

The trick (relevant to the original question here) would be to get the layers in the reverse order. A fun demo is to freeze a solid bottom layer of something that is darkly colored and less-dense (when it's liquid, but slightly more dense as a solid), then put a liquid that is pale, transparent on top. If the glass temperature, layer temperatures, and densities are right, one can watch wisps of the bottom layer rise high in the top layer. DMacks (talk)

Now that I think about it, we used some trick to get the "less dense liquid, as solid" to stay down...can't remember if we embedded a small weight in it, or somehow anchored it. DMacks (talk) 16:02, 17 October 2022 (UTC)

Ships with no atmosphere

If Earth's atmosphere disappeared suddenly, would steel ships continue floating in water? (The role of air isn't addressed in Buoyancy.) On one hand, I think "no", because the vacuum would have essentially no mass, and I think the air filling the ship helps to make the whole thing lighter in relation to the water, and the ship's slight buoyancy in air makes it weigh less than it would in a vacuum. On the other hand, I think "yes", because the water would weigh much more than the vacuum, and the weight of ship + vacuum would still be less than the weight of the water that fills the same volume. 175.39.61.121 (talk) 21:00, 12 October 2022 (UTC)

The whole thing would be moot, water can not stay in liquid state in the vacuum. It will either freeze or become vapor. Cambalachero (talk) 21:24, 12 October 2022 (UTC)

While it's technically true that you can't have liquid water in equilibrium at zero pressure, the triple point pressure of fresh water is only about 612 Pa, less than a hundredth of an atmosphere, and it goes down as you add salt. I conclude that, if all the air suddenly vanished, the oceans would start to boil, but not very fast. There would be plenty of time to observe a ship floating. --Trovatore (talk) 06:44, 13 October 2022 (UTC)

I think any potential observer would have more important things to worry about than watching ships float.... --User:Khajidha (talk) (contributions) 17:25, 13 October 2022 (UTC)

"air filling the ship helps to make the whole thing lighter": Why would that be? I believe if you created a vacuum in a dome above water and put an empty ship inside (completely empty, without air) its walls would crumble with the water pressure outside. It was not designed for that. Empirical data would be needed to determine if this is right. Bumptump (talk) 22:05, 12 October 2022 (UTC)

OK, but the answer is that yes they would float. It isn't the air that makes them float it is the absence of heavy stuff. Greglocock (talk) 22:17, 12 October 2022 (UTC)

A vacuum tube floats in water. So, an object with as much of a vacuum as we can create on earth still floats. This is not about having a vacuum inside the ship. 97.82.165.112 (talk) 00:51, 13 October 2022 (UTC)

Actually, the air inside a ship has weight, so it makes the ship heavier. When the air is pumped out of an airtight-sealed floating container, it rises ever so slightly.  --Lambiam 05:27, 13 October 2022 (UTC)

The answer may be more complicated than it looks at the first glance. Air actually adds to buoyancy – the above-water parts of a ship experience buoyancy force (upwards) from air in which they are immersed. On the other hand, air also exerts pressure downwards on parts, whose lower surface is under water (mostly the bottom of the hull). So, if the whole ship is made of materials heavier than air, then disappearing of air will make the ship's draft to grow, the hull will sit deeper in water. However, if you mount a helium or hydrogen tanks (baloons) in it, they will be adding net buoyancy in air and net weight in vacuum. I can't estimate in my head what size the balloon should be compared to the hull itself to reduce the ship's draft on atmosphere removal. Let's imagine a normal floating toy balloon with a small weight attached to it. You can choose the weight small so that it floats just by touching the water surface. If you remove air, the balloon would fall and the weight would immerse in water. However, you cant make such 'ship' to sink this way: if the balloon's buoyancy force in air is (almost) enough to fly with the weight, it obviously will prevent the weight from sinking in water. --CiaPan (talk) 06:58, 13 October 2022 (UTC)

It can go either way. The structure of the ship above the waterline (excluding air-filled spaces) experiences buoyancy from the air, which will disappear in a vacuum. In the air-filled spaces below the waterline, air provides less bouyancy than vacuum. If the volume of the ship above the waterline is less than the volume of air below the waterline, the ship will rise when put in a vacuum. In a fully metal ship this is always the case, or it wouldn't float. If however the ship is loaded with something less dense than water (like oil), the ship may sink a little.

If the vacuum is high enough, the top few decimetres of water will be boiling, until it freezes. The vapour bubbles lower the density of the water, which can cause the ship to sink a bit, but not much. PiusImpavidus (talk) 10:27, 13 October 2022 (UTC)

• If we want to spherical cow this and ignore all of the complicating factors of the oceans boiling away and all the rest, of course the ships float. They float better, because air is matter and matter has mass. A ship filled with air is heavier than a ship filled with nothing. The ship filled with nothing will thus displace a smaller volume of water than a ship filled with air will. It will thus float higher in the water. --Jayron32 11:00, 13 October 2022 (UTC)

  Just to run this into the ground, that argument applies only to the part of the air in the ship that's below the waterline. The weight of the air above the waterline is cancelled by the buoyancy due to the air outside the ship. --Trovatore (talk) 16:41, 13 October 2022 (UTC)

  That's true, but because the ship is less buoyant in air than it is in water, there will always be (in air) a volume inside the ship which has air below the waterline. If we replace that space with vacuum, it weighs less. That means the volume of the steel+void now has a lower mass than a similar volume of water than did the volume of steel+air, and as such, will displace less water. Thus, the ship floats higher in the water, QED. --Jayron32 12:21, 14 October 2022 (UTC)

Air in a ship makes it heavier, not lighter. We haven't found or made any cavorite yet unfortunately! NadVolum (talk) 12:44, 17 October 2022 (UTC)