Could you increase your speed with f-iron?

[Sythrin]

It was stated that when you change your weight mid air, there is some comverging of energy.

Did Wax underuse his power?

Could you do a dash (the martial arts fast step) with high mass. Mit dash decrease drasticly, and in that case dash faster and farther than normally?

Because the energy in the air would be converged to speed?

[Treamayne]

6 minutes ago, Sythrin said:

It was stated that when you change your weight mid air, there is some comverging of energy.

Did Wax underuse his power?

Could you do a dash (the martial arts fast step) with high mass. Mit dash decrease drasticly, and in that case dash faster and farther than normally?

Because the energy in the air would be converged to speed?

@DrPhysics Can weigh in here but my understanding: 

 F=MA (Force =  Mass * Acceleration) which also means that given the same Force, as Mass decreases, Acceleration increases. This works in the air because his Push force is a constant aournd which the changing Mass and Acceleration affect one another. Running on the ground (or simply moving a limb) is the province of F-Steel, because F=MA is constantly changing as the Force generated by the Feruchemist (Wax) is always in flux as they interact with the ground and environment. Since Skimmers are actually storing and tapping Mass the interactions and results are different that Steel which appears to mostly affect fast-twitch muscle function for the Feruchemist themselves. 

 

[Sythrin]

2 hours ago, Treamayne said:

@DrPhysics Can weigh in here but my understanding: 

 F=MA (Force =  Mass * Acceleration) which also means that given the same Force, as Mass decreases, Acceleration increases. This works in the air because his Push force is a constant aournd which the changing Mass and Acceleration affect one another. Running on the ground (or simply moving a limb) is the province of F-Steel, because F=MA is constantly changing as the Force generated by the Feruchemist (Wax) is always in flux as they interact with the ground and environment. Since Skimmers are actually storing and tapping Mass the interactions and results are different that Steel which appears to mostly affect fast-twitch muscle function for the Feruchemist themselves.

So... you support my thought?

[DrPhysics he/him]

6 hours ago, Sythrin said:

It was stated that when you change your weight mid air, there is some comverging of energy.

It's momentum that is conserved (mass times velocity).

 

6 hours ago, Sythrin said:

Could you do a dash (the martial arts fast step) with high mass. Mit dash decrease drasticly, and in that case dash faster and farther than normally?

It depends on how quickly you can store/tap mass. If you could drop to say half weight in the time that it takes you to do a dash, you'd increase your distance traveled in that time by a square root of 2 (assuming you stored mass at a constant rate). However, if you could only drop your weight 10%, you'd only travel ~7% farther. (10/sqrt(2))

However, you'd want to be able to go back to normal (or at least close to normal) weight before your attack landed. The knock back of a punch is proportional to the momentum of the punch, so at half weight you'd have to get your fist moving at double speed to have the same impact, and strength seems to scale with mass in the stories in such a way that you still move your body at roughly a normal speed.

 

Therefore, whether or not it is an effective tactic depends entirely on how quickly you can store/tap mass, and based on descriptions in the book, it probably can't happen that quickly (at least compared to the time scale of a dash).

[Sophrosyne He/any]

I think the fact that wax does do this with A-steel & F-Iron but not any other time is evidence that you can't tap and store fast enough for it to be practical any other time. I mean when you think about it you don't spend long air born when you're running and none at all when walking or jogging by definition.

I also think intent could be abit of an issue here. If you're specifically wanting to go faster while using f-Iron that might muck up the whole thing?

[Sythrin]

7 hours ago, DrPhysics said:

Therefore, whether or not it is an effective tactic depends entirely on how quickly you can store/tap mass, and based on descriptions in the book, it probably can't happen that quickly (at least compared to the time scale of a dash).

Perhaps with a spike or medallion in mental speed to coordinate it better?

F-Steel supposedly does allow one too to increase tap and burn. But that feels more redundant to use as Steel already provides speed.

[DrPhysics he/him]

15 hours ago, Sythrin said:

F-Steel supposedly does allow one too to increase tap and burn. But that feels more redundant to use as Steel already provides speed.

Yeah, using steel directly would work better than trying to improve managing mass.

[Duxredux he/him]

On 8/17/2025 at 12:40 AM, Sophrosyne said:

I think the fact that wax does do this with A-steel & F-Iron but not any other time is evidence that you can't tap and store fast enough for it to be practical any other time. I mean when you think about it you don't spend long air born when you're running and none at all when walking or jogging by definition.

I disagree on using Wax's movement as a basis for plausibility because he was a master Coinshot and used Steelpushes for the rapid movements that dashing would be useful for. BoM in the very beginning of the attack on their train he deflects bullets aimed at Marasi and rockets into the hallway by Pushing off the side of the car to slam into the wall before shooting the bandits. Keepers similarly had a simpler and more scalable speed technique available to them as well.

On 8/16/2025 at 7:44 PM, DrPhysics said:

However, you'd want to be able to go back to normal (or at least close to normal) weight before your attack landed. The knock back of a punch is proportional to the momentum of the punch, so at half weight you'd have to get your fist moving at double speed to have the same impact, and strength seems to scale with mass in the stories in such a way that you still move your body at roughly a normal speed.

I'm also iffy on if striking would be the way to go. That math checks out, but a Skimmer probably would be better off falling on their target. Rapid lunge by dropping weight, grab on like a koala and then tap to force them to the ground. Alternately step on their foot, knee, or head weighing several times normal weight.

 

First time we might see somone take the time to master this kind of technique would be when they don't have an obviously simpler method to accomplish the same task and a reason to learn it - either a Skimmer Ferring melee combatant or Malwish commandos in Era 3 or 4 with weight Medallions.

[Nitpicking]

On 8/16/2025 at 6:44 PM, DrPhysics said:

It's momentum that is conserved (mass times velocity).

Both momentum and energy are conserved in real physics. Feruchemy cheats by grabbing energy from another dimension.

[Treamayne]

8 hours ago, Duxredux said:

First time we might see somone take the time to master this kind of technique would be when they don't have an obviously simpler method to accomplish the same task and a reason to learn it - either a Skimmer Ferring melee combatant or Malwish commandos in Era 3 or 4 with weight Medallions.

Or, Sazed in HoA:

Spoiler

HoA Ch 78:

Quote

TenSoon backed up beside Sazed. “I apologize,” the dog growled. “This isn’t much of a rescue.”

“Oh, I don’t know,” Sazed said with a smile, the Fifths surrounding them. “You needn’t give up so quickly, I think.”

The Fifths charged, and Sazed tapped iron from the grate beneath his bare feet. Immediately, his body grew several times heavier than normal, and he grabbed a kandra guard by the arms.

Then fell on him.

Sazed always said he wasn’t a warrior. However, the number of times he’d said that, then been forced to fight anyway, made him think he was losing that excuse. The truth was, he’d been in far more battles over the last few years than he felt he had any right to have survived.

Either way, he knew some rudimentary moves—and, with both Feruchemy and surprise to aid him, that was about all he needed. Tapping weight increased the density of his body and of his bones, keeping him from damaging himself as he collapsed on top of the soldier. Sazed felt a satisfying crack as they hit the grate, Sazed’s greatly increased weight crushing the kandra guard’s bones. They used stone True Bodies, but even that wasn’t enough.

 

 

[DrPhysics he/him]

22 hours ago, Nitpicking said:

Both momentum and energy are conserved in real physics.

It always depends on the system. One can be conserved, or both, or neither. 

[Isilel]

On 8/16/2025 at 9:09 PM, Sythrin said:

Could you do a dash (the martial arts fast step) with high mass. Mit dash decrease drasticly, and in that case dash faster and farther than normally?

 

I don't know about dash, but reading Era 2 I thought that Iron Ferrings should be able to train themselves to run faster and jump farther by pushing off at their normal weight and reducing it in the air. Also, to do Batman glide when jumping from buildings wearing squirrel suits.

It would be even more useful when riding a bicycle or a roller, but Scadrial, sadly, doesn't seem to have either so far.

[Nitpicking]

12 hours ago, DrPhysics said:

It always depends on the system. One can be conserved, or both, or neither. 

Can you name a system in which momentum is not conserved? That doesn't describe Newtonian, quantum, or relativistic universes, so ....

Barring giant quantum fluctuations, energy is also always conserved. Note that I did not write "kinetic energy" above.

[DrPhysics he/him]

1 hour ago, Nitpicking said:

Can you name a system in which momentum is not conserved?

Any system that experiences an external force. Examples: sliding to a stop. Bouncing off a wall.

 

Also, energy is only conserved in isolated systems. Sure, energy is conserved on a universal scale (barring dark energy), but do I really care for the system I'm trying to model?

[Sophrosyne He/any]

On 8/18/2025 at 10:51 AM, Duxredux said:

I disagree on using Wax's movement as a basis for plausibility because he was a master Coinshot and used Steelpushes for the rapid movements that dashing would be useful for. BoM in the very beginning of the attack on their train he deflects bullets aimed at Marasi and rockets into the hallway by Pushing off the side of the car to slam into the wall before shooting the bandits. Keepers similarly had a simpler and more scalable speed technique available to them as well.

So, I'm saying that Wax not doing or even really trying this is evidence his using A-Steel as much as he does weaken this evidence, but it remains none the less, even after being directly informed that this was possible, he doesn't do it or even attempt, and I don't think that should be disregarded.


That said this is likely a Doylist thing in that Brandon just didn't think about it and or didn't want it in his system because it DOES step on f-Steels toes. It's also pretty granular, even for a Sanderson magic system.
 

[Nitpicking]

On 8/19/2025 at 10:08 PM, DrPhysics said:

Any system that experiences an external force. Examples: sliding to a stop. Bouncing off a wall.

 

Also, energy is only conserved in isolated systems. Sure, energy is conserved on a universal scale (barring dark energy), but do I really care for the system I'm trying to model?

"System" in this context means "closed system." So ... yeah, both momentum and energy are in all cases conserved. What you appear to be saying here is that momentum/energy can be transferred to other entities in a larger system. True but not relevant. For instance, when (for instance) a sliding object comes to a stop, it's transferring momentum to the floor, and in turn to the Earth (or presumably to Roshar or Sel). (We'll ignore rotational movement for the time being.)

In any interaction, momentum and energy are conserved.

[DrPhysics he/him]

4 hours ago, Nitpicking said:

"System" in this context means "closed system."

No. This is one of the most common misconceptions and where I have to spend most of my time when I start energy and momentum with my students (note: I'm a physics professor with a PhD who has taught for over a decade).

"System" is the system of objects you care about (i.e. The ones you are doing math with). It is not closed, unless you can prove or otherwise reasonably assume that it is.

 

To say that we always have a closed system is both pedantic and useless. When sliding to a stop, I'm not going to worry about how that changes the rotation of the planet (which we can't ignore because running across the surface technically gives you angular momentum, not linear momentum). It's also not how physicists talk about/work with momentum and energy.

This discussion has gotten off topic, so ifnyou have questions, we can open up another thread/engage somewhere else. If you want to continue arguing whether or not momentum is conserved, I'm not going to respond.

Edited August 21, 2025 by DrPhysics
Clarity

[Nitpicking]

Agreed,  time to drop the subject.