Effects of Iron Feruchemy

[Moogle]

Well, your falling speed is still the same because gravity just works that way. Your horizontal velocity wouldn't really change either because you are not exerting force horizontally in midair. Air resistance shouldn't really change much.

Is it supposed to do anything? Physics is not my forte.

 

Air resistance would matter, if you stored 90% of your weight or so (I forget the limit). Not sure to what degree, but I'd expect your horizontal velocity to slow down significantly, and you'd fall slower. It'd be a good idea to tap weight as you were falling off a building.

[natc]

Air resistance would matter, if you stored 90% of your weight or so (I forget the limit). Not sure to what degree, but I'd expect your horizontal velocity to slow down significantly, and you'd fall slower. It'd be a good idea to tap weight as you were falling off a building.

I was thinking in terms of final displacement, since we were all thinking of ways to abuse the weight shifts lately. If you fall slower but also move less sideways would there even be a difference?

. . . wait, you weren't accelerating horizontally anyway, were you? Why does the weight matter in that direction when air resistance is the only force? You wouldn't think mass has a direct effect on air resistance...

I should go back and fish out the textbook now.

[Moogle]

I was thinking in terms of final displacement, since we were all thinking of ways to abuse the weight shifts lately. If you fall slower but also move less sideways would there even be a difference?

 

Yes, there would be, since there's no acceleration on you horizontally. You'd eventually come to a near stop horizontally while still falling vertically.

 

Here is a nice image from Wikipedia:

Caption: Trajectories of three objects thrown at the same angle (70°). The black object does not experience any form of drag and moves along a parabola. The blue object experiences Stokes' drag, and the green object Newton drag.

 

. . . wait, you weren't accelerating horizontally anyway, were you? Why does the weight matter in that direction when air resistance is the only force? You wouldn't think mass has a direct effect on air resistance...

 

Weight matters because air resistance is, in effect, you pushing a bunch of air molecules out of the way. Being heavier will cause you to lose less speed for each air molecule hit.

Edited January 19, 2015 by Moogle

[natc]

Ah right. Heavier things resist change in velocity more.

I think it's clicking in my mind now. Thanks to Moogle and a pack of cola.

[dchudz]

There's lots of textual evidence above that your velocity is preserved (not momentum) when using a metalmind to change mass. But then in Shadows of Self, Ch 24, there's this:

He decreased his weight, filling his metalmind. As always, though the change was sometimes subtle, this increased his velocity. If he decreased his weight while in motion, he got a little burst of speed.

 

This seems to suggest something like momentum is conserved, and seems inconsistent with quotes people have above where nothing like this happens.

 

What's going on? Is it possible to resolve the inconsistency?

Edited November 29, 2015 by dchudz

[Stormgate]

Sanderson is not a physicist, he is an author, and this made sense to him.

[Voidus]

Sanderson is not a physicist, he is an author, and this made sense to him.

He does tend to consult with and research issues like this when writing about them though. Hence why many of us love his books so much, they're incredibly scientifically consistent for fantasy works.

[dchudz]

He does tend to consult with and research issues like this when writing about them though. Hence why many of us love his books so much, they're incredibly scientifically consistent for fantasy works.

 

Exactly. The world-building is a lot of what's interesting about fantasy, "how does this magical system work?" is the most important part of that here.

[Stormgate]

Yes, but any author who goes into this much detail about this is thinking about it too hard, and should be writing.

Unless it is a major plot point

Edited November 29, 2015 by Stormgate

[Oversleep]

Well, Feruchemy at its basis breaks laws of physics. Take iron for example.

First situation: If you are standing at ground level, h=0m and you weight 80 kg and then you use the elevator to get at h=10m, the difference is in gravitational energy - you moved 10 m away from the center of gravity and used energy to carry those 80kg for 10m.

Second situation: Well, now you also weight 80 kg, but suppose you store 10kg of mass, so your weight is 70 kg (the weight of ironmind doesn't count for simplifying the calculations). Now you also go to the h=10m, but you used only as much energy as needed to move 70kg from the center of gravity. The catch is that you can stop storing iron and now you weight 80 kg. (It gets even worse if you tap)

So in both situations you weighted the same at the beginning and at the end, but in the second situation less energy was required to move the same distance. Since there is no loss of energy in Feruchemy (assuming you tap a the same rate you stored), where does energy which moved the rest of 10kg come from?

Conclusions: let's just stick with Law of Conservation of Momentum, since it looks like both Allomancy and Feruchemy obey them.

Edit: also, tapping iron actually would make a Skimmer fall faster. A little. When we're talking about huge speeds, like jumping with parachute it would become a little visible.
It is not because of gravity, but because of air resistance vs gravity. Much like storing iron makes you fall lighter, tapping it would also move the point of balance between air resistance and gravity. There is a point in which your fall stop accelerating because air resistance balances the gravity force. Remember Felix Baumgartner?

Edited November 29, 2015 by Oversleep

[dchudz]

Oversleep-- I don't think "momentum is conserved" works, due to some of the examples above. For example (quoted by ulyssessword above as from HoA ch. 78 pp 678):

[sazed] slammed the hammer down on a kandra shoulder, growing heavier to add to the momentum of the blow.

 

 

 

If momentum were conserved, growing heavier wouldn't add momentum to the blow (and would in fact decrease kinetic energy).

 

I'm now thinking that maybe the conserved quantity is kinetic energy. Then increasing your mass does add momentum to the blow. If m1 and v1 are the original mass and speed, and m2 and v2 are the new mass and speed (after changing mass), 

 

m1*v1^2 = m2*v2^2

 

so

 

v2 = sqrt(m1/m2)*v1.

 

Then momentum (mv) is increased when increasing mass.

 

This looks to me like it explains everything that happens when changing mass reasonably well. Decreasing mass to slow down while falling looks like an exception, but (as some folks discussed above) air resistance explains that: it's plausible you'd slow down after initially speeding up a lot. (It's a bit odd that the text doesn't mention the initial speeding-up, though. I guess maybe people don't notice it, since they're already speeding up from the fall?)

[Edit: As someone mentioned above, "which reference frame?" is still a problem / open question.]

Edited November 30, 2015 by dchudz