Skimmer Buoyancy Unchanged?

[+Lewis Nethur He/Him]

Sorry if this has been previously discussed, but I couldn't find it on the forums or the Coppermind.

I was thinking, Skimmers can manipulate some aspect of their weight/mass/density to, for lack of a better term, become weightless.  I can accept that, however, what strikes me as seriously odd is that Skimmers storing "all" of the weight/mass/density that they possibly can don't float in air; IE: Skimmers' buoyancy remains unchanged when they're storing/tapping.  A helium balloon isn't "weightless", it's just, on average, less dense than the atmosphere around it (at sea level), causing it to accelerate upwards.  I feel like it would have been commented on at some point if Wax could float into the air (without steel pushing or jumping) by simply stripping naked and storing iron.  Further, I would suppose that would mean that, despite not being able to "dive" into a body of water, due to being weightless while naked and storing iron, he would still tend to "float" at the same depth once submerged?  Do we have any in-book scenes showing Wax walk on water?  If Wax went to Roshar, where the atmosphere is less dense, would he not be able to become completely "weightless?"

So what does this all mean?  At its simplest, this means that, despite the fact that the volume of fluid displaced by a Skimmer is unchanged while they are storing iron, the buoyant force exerted on them by the fluid they are displacing decreases in direct proportion to the amount of iron they are storing.  On its face, this seems like it makes absolutely no sense to me; it would be like two wooden barrels floating at the same depth in water despite one being empty and the other being filled with lead.

Does anyone have any insight on how this conundrum could be reconciled?  Every explanation I've come up with has seemed contrived or absurdly convoluted and I feel like Sanderson has a pretty solid track record for not making in-book physics behave in silly or unexplainable ways.

[Spoolofwhool]

First of all, a Skimmer cannot store all of their weight (mass) at a given point. It's just a physical limitation of feruchemy, the same way you cannot store all your physical strength or physical speed. Wax usually only goes around at 3/4 normal weight (mass). Also, even if you could store 100%, you would never have a total weight of 0 N because there would still be a force of gravity on your metalminds.

Secondly, skimming does affect buoyancy. This is shown in WoA when Sazed stores most of his weight (mass) then steps off of a cliff and floats to the bottom, because his terminal velocity is so low due to the force of gravity and buoyancy force of air being a lot closer together in magnitude.

This should solve your conundrum. 

Edited November 28, 2016 by Spoolofwhool

[Jofwu he/him]

31 minutes ago, Spoolofwhool said:

This is shown in WoA when Sazed stores most of his weight (mass) then steps off of a cliff and floats to the bottom, because his terminal velocity is so low due to the force of gravity and buoyancy force of air being a lot closer together in magnitude.

I don't think that has much to do with buoyancy. Terminal velocity is the speed where drag force equals weight. Drag has nothing to do with buoyancy and isn't affected by storing weight/mass. Sazed fell slowly simply because lower weight and same body equals lower terminal velocity.

Buoyancy is something very different. It COULD come into play if Sazed decreased his density by a significant amount. Considering there are no cases of Skimmers becoming weather balloons, I don't think they can lower their density so much that buoyancy comes into play significantly when in the air. Water is another matter, because it's significantly denser.

To answer OP, I think your answer is in the fact that there's a limit on how much weight they can store. I don't know if we have an idea on what that limit is, but I doubt it's enough to allow floating or walking on water.

Edited November 28, 2016 by jofwu

[Spoolofwhool]

22 minutes ago, jofwu said:

I don't think that has much to do with buoyancy. Terminal velocity is the speed where drag force equals weight. Drag has nothing to do with buoyancy and isn't affected by storing weight/mass. Sazed fell slowly simply because lower weight and same body equals lower terminal velocity.

Buoyancy does play a part in terminal velocity. Terminal velocity is, as you say, when the net force on an object is 0, and buoyancy is one of those forces, just like weight and drag force. I am not sure whether it is a relevant amount though. You are correct though that buoyancy is an insignificant factor in air, so my bad.

Buoyancy also doesn't really have anything to do with density, unless you're saying he would be changing his volume at the same time.

In any case, to readdress the OP with some math, assuming that your average skimmer has a regular body weight of 70 kg, giving a volume of 0.07 m^3, in order to actually start floating, assuming buoyancy is the only factor, they would have to be storing >99.9% of their weight, which I don't think is possible.

[Jofwu he/him]

Right, we're on the same page. I was just arguing that it's insignificant in the case with Sazed.

And density matters as that's the weight that the buoyant force has to counteract. Though that's perhaps a roundabout way to put it. :-)

Some more math... If you want to stand on water with just the bottom half of your feet submerged... That's perhaps 0.001 m^3 which is 1 kg of water displaced. You'd need to decrease your weight by 98.5%. Not as much as you need to float in the air, but still a tall order.

Edited November 29, 2016 by jofwu

[+Lewis Nethur He/Him]

I believe Wax stores weight and uses a shotgun to fly around at some point...I don't have the books available at the moment, however, if someone can recall or figure out an estimate of how high he can go with this method, we can get a reasonable estimate for what percentage of his weight he can store.  Considering how much a shotgun, boots, and a full set of clothes are likely to weigh, off the top of my head, I don't think it would be unreasonable for him to be able to store upwards of 98% of his weight.

Now, I'll grant you that "walking" on water using this method would be laughably ineffective unless you had a running start before you started storing weight or touched the water's surface.  It would be more like running and splashing in place than actually going anywhere.  However, in the event of an emergency, Wax could be used as a flotation device for several adults 

[Spoolofwhool]

3 hours ago, hwiles said:

I believe Wax stores weight and uses a shotgun to fly around at some point...I don't have the books available at the moment, however, if someone can recall or figure out an estimate of how high he can go with this method, we can get a reasonable estimate for what percentage of his weight he can store.  Considering how much a shotgun, boots, and a full set of clothes are likely to weigh, off the top of my head, I don't think it would be unreasonable for him to be able to store upwards of 98% of his weight.

Now, I'll grant you that "walking" on water using this method would be laughably ineffective unless you had a running start before you started storing weight or touched the water's surface.  It would be more like running and splashing in place than actually going anywhere.  However, in the event of an emergency, Wax could be used as a flotation device for several adults 

I don't recall a single instance of Wax using a shotgun blast to push himself upwards. What I do recall him doing is either shooting the shotgun and pushing off the pellets using allomancy, or using the shotgun to push himself sideways while falling, in which case his weight ceases to become a factor, and his mass just impacts the momentum shift.

[cometaryorbit]

I think the reason we don't see Skimmer-balloons is that the weight of clothes, metalminds, and equipment is too much for that.

The density of the human body is close to that of water, ~ 1000 kilograms per cubic meter. So a 100 kg (rather large) human will have a volume of about 0.1 cubic meter, and will displace that much air. Density of air at standard conditions is about 1.2 kg per cubic meter, so that's just 0.12 kg of air - a bit over four ounces.