DrPhysics

On 6/14/2026 at 5:44 PM, Chaos said:

A = 1/2 tau*r^2 is very symmetric with the kinetic energy formula. That 1/2 should be there anyway, due to integration.

@ChaosI'm going to call you out on this one (I'm a physics professor). If that's your justification, then volume of a sphere would be 1/6 tau r^3, not 2/3 tau r^3

Area integrals don't need to be symmetric with path integrals. (Please note tone of joking/sarcasm)

And solutions to the wave equation are much more satisfying to write as sin(n pi x/L). Who would want sin(n tau x/(2L))? That's a whole other constant I need to track through separation of variables.

Also, if we use tau for 2pi, what am I going to call torque, time constants, and shear stress?

On 6/13/2026 at 10:51 AM, Treamayne said:

Higgs fields naturally interact with energy density, but not physical density - though my physics education falls far short of really understanding the interactions at play. Possibly @DrPhysics will find time to weigh in (pun totally intended)

I have been summoned!

So, @alder24 is correct. The higgs field only accounts for a tiny fraction of the mass of all matter (electrons and quarks) but most mass is from binding energy. 

What he didn't talk about was what would happen if we adjusted the Higgs field strength. Here's a very important fact: electron orbitals and the size of protons and neutrons depend greatly on their wavelengths, which are also inversely proportional to their mass. If you double the higgs field, you double the electron mass, cutting its wavelength in half, which also cuts its obit in half. Since they are closer to the atoms, binding energies get much much higher, the repulsion forces between nuclei wouldn't let two atoms get that close, chemistry as we know it collapses, and organic molecules all fall apart. Oh, and doubling the higgs field would only effect the mass of the electrons and quarks, which only accounts for about 1% of the mass of a person. So, by increasing your weight by 1% through the Higgs field, you would most likely cause every single molecule in your body to fall apart.

Our best explanation for storing mass is simply somehow the magic changes your interactions with the world so that you have effectively a higher/lower total mass, without changing your molecular density or how those molecules behave.

That, and any mass you store/tap is considered to be at rest with respect to whatever you perceive "at rest" to be (otherwise the momentum stuff doesn't work).

21 hours ago, The_Lopen279 said:

we can only push directly away from the center of self.

That is true at long ranges, but there are several instances where it appears that allomancers are pushing/pulling with only part of their body. I mentioned a few in the post that @Treamayne linked, but I'll not list them here because I don't know what level of spoilers we are at. This will not be the only time you see that contradiction.

As far as breaking the glass goes, the best I could find was this paper that shot BB's at sheets of glass, and observed the breakage pattern. They found that if you had an impact energy of around 3-4 J would give glass failure at a level where the BB went through (for reference, a .22 rifle would have a 40 J impact energy). They used double-strength window glass, which would probably be comparable with a well made glass vial. We couldn't produce tempered glass for anything except curiosities (like Prince Rupert's Drop) until the late 1800's, so they wouldn't have had the tech available in the Last Empire either.

Since neither metal nor glass has much give, impact energy will be a better predictor for shattering than anything using PSI (Only the front most part of the bead would touch the glass before shattering it, so using the entire cross-section of a sphere would be a bad predictor).

So, 3-4 J does mean a velocity of around 130 m/s for the BBs, which sounds insanely fast, but it doesn't have enough energy to do much to you. A 1 cm diameter steel ball bearing would only have to travel at 45 m/s to shatter the glass, which matches your 90+ mph calculation, but it would still only have 3-4 J of impact energy, which is equivalent to a baseball going about 16 mph.

But, we have two other things to consider: (1) these speeds would be the speed of the bead relative to the glass, which would be slower than the speed of the bead towards Vin and (2) that assumes that the bead doesn't lose any energy or speed on impacting the glass, which is impossible. Doing the exact calculation for speed involves many more assumptions (thickness of the glass, speed of the vial away from Vin, cross-sectional area of the bullet, which matters if you are going through, but not for whether or not the glass breaks, ect.) than I feel comfortable assigning an exact number to, but some back of the envolope calculations suggest that the bead should lose somewhere between 25%-75% of its enery, which works out to be 45%-85% of its speed.

On 2/16/2026 at 11:44 AM, listerfeend said:

Brandon has been super cagey about whether mass==weight in the context of Feruchemy)

Since Wax speeds up when storing "weight" in the air, he is most definitely storing mass. If he was sitting how gravity attracted him, you wouldn't see the speed up. 

On 2/17/2026 at 1:11 PM, Espella said:

My assumption would be that any methods that bypass speed of light involve spiritual realm shenanigans in some way since we know time works different there.

You're right. So far, everything suggests that information/investiture/etc is bound by the speed of light within the realm it travels in.

Span reads share a connection that is separated by the physical realm. Aons connect through the spiritual realm, so the effective distance between them is zero. 

I imagine one of the ways around the Elsecalling portal that he talks about would be to cross the much shorter distance in the cognitive realm at the speed of light, popping out faster than light would have traveled in the physical realm. Another would be to enter the spiritual realm and pop out somewhere else.

On 1/4/2026 at 11:02 PM, SeekingShards said:

My first concern is if a repeller would be strong enough to push away water at any substantial depth. The rules text says it pushes away the attuned material weakly. But it also says “If the repeller is heavier, it pushes the attuned material away from it at a rate of 1 foot per round [Approximately 10 seconds].”

Physicist here. This rule works well for an RPG since it is simple to calculate and follow, but it doesn't work as a way to estimate force because the object moves at a constant speed, which just means that all of the resistive forces (like friction) balance out with the force from the attractor, giving a total force of zero. And calculating all of the resistive forces would be a nightmare for something like water. (Is it flowing across the ground? Is it in a sliding bucket? If so, what is the bucket made out of? etc.)

Everything we've seen in the books so far makes it looks like attractors work by exerting a long-range pulling force like gravity or static electricity, just attuned to a single substance. In Words of Radiance, we see one draw water out of the air, and another that collects smoke from a fire.

In a fluid, that would result in a small, pressure-like force. Pulling water out of air is easy, because air is just a loose collection of molecules. Pull on the water a little bit, it will just drift towards the attractor without having much impact on the air around it (you'd see a little bit of air drug along through collisions, but not enough that you'd expect to be able to feel a wind). Pulling air out of water is much harder because the air is actually dissolved in the water, and there are intermolecular forces bonding (albeit weakly) to the water molecules, so if you start dragging the air out, you also exert a significant force on the water, pulling it in with the air.

Using a repeller to create a pocket of air around you is incredibly difficult. Because fluids can rearrange themselves, if you create a bubble around yourself under water, you don't just have to push away the water immediately around you; you have to push away hard enough to lift all of the water above you as well. (Think about trying to lift a stack of books. Technically, you only puch on the bottom book, but that push has to be strong enough to lift the rest of the books in the stack too.) To put that into perspective, if you have a repeller attuned to air that can push hard enough to create a perfect vacuum around itself (which is much stronger than anything we see in the books), and you changed it so it could push water just as hard, your bubble would completely collapse by the time you reached 10 m below the surface. Water is very, very heavy.

On 11/11/2025 at 2:36 AM, Xanpheon said:

1. Apply multiple lashings to themselves, horizontally, in the direction of their target - accelerating them very strongly in that direction, increasing their momentum significantly.
2. While accelerating, launch a powerful push on a coin in front of them.

This should, going by the equivalent/opposite effect, act almost like a Newton's Cradle - combining the acceleratory force of the multiple times gravity with the sudden massive force of the push.

Yes, this should work.  The downward lashings would be comparable to having a strong anchor, but the forward motion would keep you closer to the coin allowing you to push harder, longer. 

However, there are two big caveats to this. One, you would up the coin's knock back, but you wouldn't speed it up very much (see this discussion: 

The coin is already moving so fast they there just isn't much time to push on it. However, once it slams into a target, it will still have all that force left behind it.

The other consideration: you don't want to miss. Hitting a solid object with a coin would hurt. (Remember Vin getting knocked back by the door?) This suggests that allomancers push with a constant power, rather than a constant force, and having all that extra power in the coin would slam into you, and it would not be fun.

If it helps, here's a thread I wrote a while ago that summarizes everything we know about pushing physics, and it might help with some of your theorizing:

Since many of the others in this thread have stated that they aren't physics people, I thought I'd start by saying I am one. I'm a Professor with a PhD that has been teaching physics for over a decade.

On 10/20/2025 at 2:16 PM, IHadAThought said:

There might be a problem with that for the objects moving towards each other, they have to ignore all other sources of gravity, or at least have the same pull during the entire time. They would have to ignore the planets gravitational pull for that to work, which would launch them out of the planet and into space. The only way for that to work there would be if they both objects retained their gravitational pull to the planet, and if that pull was the same, which it’s not. The magnetic field has different strengths at different areas, and we’re all different distances from it.

The biggest problem is that gravity can only pull.  Gravitational lashings work (sort of, with some problems) by changing the direction of the connection to the planet. You're always still attracted; direction just gets funny. And here's the important part of that: it breaks Newton's third law. The planet would still be attracted to the person, even as the person was falling away from it. If pushing worked by doing gravity but backwards, we'd still have that weird force flipping element, and having a chunk of metal slam into something wouldn't throw you backwards; it would pull you towards it.

Eddy currents don't work, either. Creating eddy currents large enough to make metal attracted/repelled as described in the books would make it very hot (some of Wax's heavy pushes would have started fires) and would make the metal that was being pushed/pulled an electromagnet strong enough that we should have seen it attracting other ferromagnetic materials.

I did a post a while ago summarizing everything we know about steelpushes (link), though I didn't do much to address how they work. The way they describe being able to see atoms (as was mentioned previously) and how it affects metals more than other materials suggests that it is electromagnetic in nature (the gravitational signature from individual atoms would blur too much to see them), but there are no electromagnetic processes in our universe that could make metal behave that way without some very obvious side effects (like eddy current superheating metals that are pushed hard, or magnetized metals acting like magnets), so the only explanation we have is "Cosmere magic via a mechanism that we don't understand yet", which isn't very satisfying. Sorry. 

1 hour ago, AltonicKeys said:

So if there was a magnet hanging from the ceiling, positive pointed to the zap metal, the arc would travel towards the magnet first, and then target the ground?

Magnetic forces are perpendicular to magnetic field lines (which would point directly at the zap metal if the magnet was pointed towards the zap metal), so the moving electricity would feel no force as they flowed towards the magnet. 

1 hour ago, AltonicKeys said:

I've been trying to make a 'plausible sci-fi' look cool, while trying to stay away from magic

The problem here is that there is no physical mechanism that could explain what you want the zap metal to do (even piezoelectric materials), nor could we create one. It goes too strongly against the rules of the universe (aside: I'm a physics professor and teach this stuff pretty regularly).

Learning is great, so keep digging. 

And before when I said "magic", I really meant phenomenon unexplained by physics. Many sci-fi stories (including some hard sci-fi) have no problem pulling the "unobtanium acts like x because it just does" card. I've just taken to calling that tech magic because it lacks and/or defies explanation.

What you have is awesome. Let the rule of cool win and don't worry that it breaks the laws of the universe. 

So, what @Seonid says about capacitors is true, but you run into a problem where the capacitors will only discharge from one plate to another and you wouldn't get a big arc like you are hoping for, instead you would end up with a tazer (which uses a capacitor). Lightning arcs because the ground acts as one plate and the clouds/atmosphere as another. 

You also can't just say it uses it's energy to build up a big store of charge. That would explain the arcing (the wack breaks the hold on the excess electrons so they can fly free) but if you built up a charge big enough, the zap metal would start to be strongly attracted to any neutrally charged or opposite charged objects (think rubbing a balloon on your head and sticking it to the wall, but hundreds to thousands of times stronger.

At some point, you just need to say it's "magic".  If you ground it too much in real world physics without physics expertise, you're going to alienate anyone who does have that expertise. 

Marvel's arc reactor is a great example of this. It's essentially a magic power source. Yes, it's supposedly based in technology, but they never dive into how it works and it becomes easy to suspend disbelief when you're presented with "this machine makes power because it does" compared to "this machine makes power because of some nonsense reason that doesn't make any sense".

On the other hand, you have stories like Project Hail Mary, where he tries to describe the science behind everything and fails horribly. The story is amazing, but I almost couldn't finish it because every few pages I'd be screaming in my head "That's not how the universe works".

I think you are in a good spot with this magic system. Make sure your devices function consistently (that they follow their own rules), and don't worry so much about the physical details of how they work because there isn't anything that would do what you describe without introducing many other problems. 

21 hours ago, Treamayne said:

@DrPhysics can possibly weigh in if they have time, but my understanding is W=MG, so an object massing 10kg in 1 G would multiply by 9.80665m/s; in 1.2 G you would multiply by 11.76798 m/s for gravitational acceleration. You would perceive the object as being 12kg (as laymen think of "weight"). I don't think Connecting to a planet would change this, but a person would adapt to the local conditions given time (like exercising with a weighted vest). For an adult that "weighs" 150lbs in Cosmere Standard, they would feel as if they "weigh" 180lbs on Sel. 

That is correct. Just take the weight and multiply it by 1.2.

That said, this only applies to lifting things. Something that rolls on good wheels would feel comparably hard to push on both planets.

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.

On 7/17/2025 at 7:14 AM, alder24 said:

But as Treamayne said, Fused passive effect allows them to Lash themselves without consuming investiture, so F-iron wouldn't change a lot for them. It could only increase their maneuverability and help them turn or decelerate faster ... But it would have no effect on their acceleration and investiture consumption rate.

Maneuverability and deceleration are also accelerations and would not be affected by changing mass. You could make an argument where decreasing mass might help you slow down faster by reducing your terminal velocity and air resistance, but there's a trade-off with momentum temporarily speeding you up. It definitely won't help with turning, because drag forces only act in the direction opposite travel.

On 8/18/2025 at 12:03 PM, Heilven said:

We know that depictions of g-force or forces due to lashings are due to the underlying mechanics not being fully thought out (to the general relativistic level that is. That's pretty far to think something out).

The depictions we see are wrong with good old Newtonian gravity as well. It's unclear whether that was a mistake by Sanderson or done for audience expectations, but the g-force effects only apply when something resists the pull of gravity.

Put a hole in the bottom of a water bottle and drop it. While in free-fall, no water will come out because there is no pressure build up due to the container holding up the water.

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?

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. 

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.

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).

If they were, they wouldn't stay there long...

27 minutes ago, Ashkaloda said:

I'm pretty solid at physics, but there's something I'm not getting here. I can see how you could propel the skimmer in this way, but how would that affect the propulsion of the airship? 

You'd have to strap in the skimmer. If you can create a force on the skimmer (or propel them in any way) they can in turn exert a force on the ship. It's like strapping someone with a jetpack onto the top of the ship. The jetpack pushes the person who in turn pushes the ship.

5 hours ago, KaladinsSenseOfHumorSpren said:

The only thing is that the ship is going to be much heavier than the skimmer, so even storing to the point of air-like mass, then you wouldn't speed it up that much.

The force depends on (1) how quickly the skimmer can store mass and (2) how fast the ship is going but does not depend on the skimmers mass, unless that somehow impacts #1.

So, if the ship has a decent jump start and/or the skimmer can cycle quickly, they would be able to create a significant push.

Our real rockets work by ejecting a tiny mass very very quickly, so could this one.

I spend too much time thinking about physics in the Cosmere and invented a form of propulsion. 

Background:

We know from the books and Brandon himself that momentum is conserved when a feruchemist changes their mass (weight) in the books.

Quote

Brandon Sanderson:

1). Wax changing his weight doesn't change the pull of gravity on him, or the rate at which he falls. 2) He DOES follow the laws of conservation of momentum. Source:Reddit

There's been a few discussion here on the shard about how this breaks down with reference frames, but every situation in the books works if you assume the mass that is stored/tapped is at rest with respect to the skimmer's reference frame. That's why Wax getting heavy on a train doesn't slow him down, because he sees himself as sitting still on the train.

In the books, if you decrease mass while flying through the air, you will speed up. If you increase mass, you will slow down. That works if you assume the mass you are storing/tapping moves at the speed of the room you are in. Really, what you are doing is stopping the mass when you store it, giving the momentum that it had to you, so you speed up. When you tap the mass, you have to speed it up to your speed, making you slow down. 

However, if you tap mass that is already moving at your speed, you don't change speed (this is why when Wax gets really heavy while sitting still, he doesn't slow down relative to the rotation of Scadrial).

Propulsion:

Now imagine that you could change reference frames either using perception (opening/closing a window with some sort of metal mantra or concentration aid) or using Connection. You could do propulsion this way:

1. Put a skimmer on an airship (or rocket) and convince them that they are moving in the airship/are connected to the ground.
2. They store mass that isn't moving in the reference frame of the planet, speeding them (and the ship) up.
3. Close the windows/change Connection so that the skimmer is convinced that they are at rest with respect to the ship,
4. Tap mass that is moving at the speed of the ship and therefore won't slow it down.
5. Repeat from step 1

This works similar to a rocket - you jettison mass in a way that increases your momentum - but has the advantage that you don't have to carry all the fuel with you. If you could repeat this process fast enough, after a small starting push any skimmer would be able to reach orbit (or beyond) for very little fuel cost, instead just alternating storing/tapping weight and their perception of their surroundings.

We can even come up with the relevant thrust equation:

Force = change in mass/change in time * speed of the ship (F=-dm/dt*v) (negative denotes that force is opposite velocity)

Launching the rocket - aka the Bendalloy Railgun

If we can find ways to manipulate Connection, you could launch the ship by having it enter a bendalloy time bubble (while attached to a sturdy rail so that it doesn't deflect) while the bubble is connected to the planet, making the rocket speed up. Then you switch the connection so that the thing making the bubble is connected to the rocket and the bubble travels with it, so that the rocket doesn't leave the bubble until it reaches its destination and you attach the bubbles to the new planet and let the rocket slow down as it exits each one.

I'm imagining a charged ettmetal grenade in a magnetic casing that you could just drop onto the ship as it goes by, or use a system that lets you turn an iron pull on/off to bring them in.

Stacking bubbles (so that you enter several) could increase the speed further.

Matter seems to interact differently from light. Based on the way the bubbles are described as looking in the book, the edge of the bubble essentially has to absorb all the photos hitting either side, then re-emit photons out the other side in the proper color and amount so that it appears to be clear. 

Matter seems to go through without getting created/destroyed, it's just sped up/slowed down appropriately and deflected.

We haven't seen any liquids/gasses cross the line (the explosion broke the bubble when it hit, the slow mo happened while it was outside).

The linked post was my attempt to describe what wind does based on what we know about the physics.

I'm remembering (but can't find the source, so take this with a grain of salt) that solid objects are either all in or all out of the bubbles. It's cognitive aspect controls that (I'm in, vs I'm out), and the weird deflections happen as they suddenly end up in the other side.

I don't know how a stream of water would view itself. Most water traveling through the air like that will break into disconnected droplets, and I imagine that those would cross like bullets and other items. 

What a connected stream would do is much harder to predict. The easiest solution would be the all in/all out method. Bubble boundaries would stop at the edge of flowing water. The rare, unbroken streams would be thrown out all at once, possibly deforming the edge of the bubble so that it wraps around the stream and keeps it all on the same side.

The thread linked by @Duxredux addresses the heat, but doesn't address what would happen with a laser.

A WOB/story could change this in the future, but if lasers act like regular light crossing a boundary, they will go through without bending (stuff outside a bubble looks normal except for a barely noticeable shimmer) and will be scaled to match the proper brightness/intensity (otherwise it would be very dark in speed bubbles and very bright in slow bubbles).

That said, the time difference would make it hard to do anything, because you have to hold the laser still on the target for long enough. As an example, there are green lasers that you can carry around in your pocket that are bright enough to pop a balloon if you you can shine the laser in one spot for a second (yes, there are much more powerful lasers, but this one works for the example). In order to pop the balloon through a factor of 10 slow down (either from regular time into a slow bubble or from a speed bubble to regular time), you'd have to hold the laser in place for a full 10 seconds of fast time to deliver the one second in slow time needed to pop the balloon.

For reference, commercially available laser cutters can cut through 1/4 inch plywood at 10-20 mm/s. More powerful lasers would need a pretty big speed bubble, and we have no idea in how electricity would handle going into a bubble.

5 hours ago, idanstark42 said:

Here is my calculations (for giant with luminosity 10^5):

Because we define a year as once around our sun (1 solar mass) at a distance of 1 AU, if you stick in those units G=4*pi^2 (try plugging those ones into the period equation and solving for G). It makes entering the values into a calculator much easier.

That said, I didn't include the 2pi when I crunched the numbers, so I also get 1660 years for the 400 AU case.

5 hours ago, idanstark42 said:

If you have ideas for something more rigorous I would love to hear.

We could narrow down the intensity at surface range for the supergiant by estimating the planets albedo(which is hard), and knowing that the white dwarf primarily emits in the ultraviolet would let's us narrow down its temperature range using blackbody spectra and white dwarf properties. Beyond that, we just don't have enough information.

We also know that since Peter Ahlstrom has a background in astronomy, when they say it is a blue-white supergiant, they probably mean a class B supergiant which limits the size and luminosity more than just Blue Supergiants, which could be from class A, B or O, and your data ranges include those three classes.

On 3/25/2025 at 11:35 AM, idanstark42 said:

Any thoughts?

Something is off with your period calculations. When I run the white dwarf as 0.5 solar masses and the giant as 22.5 solar masses with a 400 AU distance, I get a period of 265 years.

The planet won't follow there period equation you used because it is at a Lagrange point, so you need to use the giant-dwarf system.

Other than that, the logic looks sound.

Once, I created a potential Taldain system (though there are several other solutions), and posted my work here:

https://www.17thshard.com/forums/topic/198004-taldain-orbital-system/

We're missing the information needed to answer this one. 

Here's the conundrum. when throwing something, we hit two limits: How fast we can move our muscles and how hard we can push. Throwing something at double speed means not only are we moving our arm twice as fast, but we also quadruple the amount of force  (work= kinetic energy -> F×d=1/2mv^2. Double the speed, you quadruple the energy and also quadruple the force since our arm still moves through the same displacement).

The WOB are all vague on this one, and we don't see many interactions from the point of view of a steel runner.

It many cases we see that the cognitive aspect plays a role, thing happen like you expect them to. We don't see clothes getting torn apart because a steelrunner accelerated out of them harder than the tensile strength of the fabric, but we dont know why. If it doesnt happen because the steelrunner has learned to limit acceleration so that they don't leave their clothes behind,  then that's a type of world where a steelrunner wouldn't be strong enough to throw something much faster than they normally would. Or, they might not leave their clothes behind because they see the clothes as part of themselves/their sped up timeline and so investiture speeds the clothes up too. In that world, a steelrunner throwing things could be incredibly deadly.

In the books, (as far as I can remember, if there's a scene i forgot call it out here) we mostly see steelrunners attack with small weapons, like a dagger, or carrying a gun. Accelerating a 6 oz dagger at 10 times speed could feel like swinging around a 10-30 pound weight (depending on the acceleration), which is heavy but doable, especially because you'd only feel the increased weight when you were changing its speed or direction. Or, if it sped up like it was part of you, you wouldn't feel any difference moving it around at 10x speed as it would at regular speed.

Since both options are feasible, we just won't know which method works with steel running until we see it from the steel runner's perspective.