Go back to around pages 3-5 for the recoil discussion. Short answer is that recoil is based on momentum. Whatever momentum the projectile acquires, the weapon acquires. Momentum is mass x final velocity. There's also some disagreement about recoil velocity, where light weapons have more "kick" than heavy ones.

The firearms industry references energy because smokeless powder has a fixed explosion velocity where the only variable is quantity (energy).

The trouble with gauss weapons that generate high velocities are power needs and structural strength. The "peak discharge battery" of SR4 gets past the first while the latter can be considered solved by nanomaterials and/or dikote.

F=MA actualy, what do you mean with "going total"?
Tachi sorry to bother you but is it true that the bullet leaves the muzzle before the recoil is felt? If so, how does it come that way?

Because the operating parts of the gun start moving and delay the transmission of the recoil until they come to a stop. And the round is moving like a bat of out hell and it takes very little time for the round to travel through the barrel. Say the round take two miliseconds to go from zero to 5 km a second over 0.5 meters. If the various parts of the gun operating system don't strike anything for those 2 ms the shooter won't feel the recoil until after the round is gone downrange.

Plausible.
My thanks.

For people arguing the thunderstruck, remember it's a heavy weapon with double the recoil modifier. If you think double modifier is double momentum that's 4x the energy using the same projectile. If you're more conservative and think it's 1.5x the momentum it's still more than 2x the energy for the same projectile.

If you start with the .50BMG as your gunpowder, non-heavy weapon starting point (~900m/s @ 18kJ) it will go to 1350m/s @ 40kJ at 1.5x momentum or 1800m/s and 72kJ at 2x momentum.

If you switch to a .50 SLAP round (@1/2 the weight according to wikipedia) using a non-heavy guass weapon you start with 1.8km/s and 36kJ. (Note, the fastest gunpowder rounds out there max out around 1.3km/s because propellant speeds are less than 2km/s and won't get a foreign body up to those speeds.)

For 1.5x momentum heavy weapon you'll see 2.7km/s and 72kJ. A 2x momentum heavy weapon version is 3.6km/s and 144kJ of energy.

I never got past AP physics, so I never learned about magnetism, but it strikes me as weird that there would be recoil. Like, if a projectile was hovering within a magnetic field, and then I used the magnetic field to fling it, wouldn't there be 0 recoil? There is an equal and opposite reaction, but it's between the projectile and the magnetic field, not whatever surface the magnet is anchored to... right?

AP phisics?
Anyway, I think the recoil is dued the action/reaction principle, the magnetic system applies a force to the round so it recives an equal and opposed force as result; however the recoil produced would be much lower than chemical propelled systems.

AP Physics is refering to the Advanced Placement course covering Physics. At least that is what I am guessing. There are AP courses for several things, chemistry, physics, calculus, etc. I am assuming it is the same elsewhere as here (Texas) where they are taken in high school and give college credit if passed. Larme was saying they either failed or had to drop the class.

Lol, no, I never failed a class. By "didn't get past" I meant that's as far as I got. Though actually, now that I remember, it wasn't AP physics, it was honors phsyics. Which is less advanced, and less rigorous. It's just the regular physics class, but for the smart kids. I really would be interested in a cursory explanation of magnetism and recoil, instead of having to look it up myself... I know there's someone on here who knows this stuff!

Well I don't know much of magnetism (I know that to be explained properly it needs alot of vectors and alot of computation on them) but I know that if you take two magnets and place one near the other with the same polarity they are going to repel each other, you can use that repulsion to "push" one by using said repulsive force without touching it, but you are going to feel tist resistance anyway, in the railgun it's the same; just think to the weels of a car, they push the car forward by pushing the ground backward the car moves because its mass is negligible compared to the earths one, howerver the ground is subject to the "recoil" of the traction and if this "recoil" excides the friction with it you lose aderence. If you apply a force to a bullet the same force is applied to you (with I mean the gun before, than discharged to you, than from you to the ground or what sustaines you), the reason why you don't fly away is that you are much more massive than the bullet, I hope you got what I'm trying to say despite my english.

We obey the laws of physics in this house, young lady! That way lies madness, squid people, and perpetual motion machines.

Regardless of the mechanism, equal and opposite reactions hold true. If you transmit a force through a mag field, the reaction is transmitted through the field back to the whatever created the field.

Gauss weapons do have somewhat less recoil than gunpowder weapons but that's because there's no unutilized kinetic energy. When you fire a gun, there is recoil from the propellant gasses. The gas can't transfer its kinetic energy completely, resulting in additional recoil above and beyond that of the projectile.

That's not saying a gauss weapon is 100% efficient, just that the losses are more along the lines of heat and ionized gases.

Two words: "bop drive".

The operative equation here is momentum, which is equal to mass times velocity, P=M*V. But you are also right that F=MA (newton's second law) is extremely relevant to this discussion.


No really this is Physics 101 stuff. Newtons 3 laws and Thermodynamics, concervation of energy and all that.

What it all boils down to is rather simple. If a projectile leaves a gun with 20 N*s of momentum, the gun will be thrown backwards with 20 N*s of momentum. The method this is done with is entirely irrelevant, be it gunpowder, magnetism, or rubber bands. Guass weapons are not immune to this effect.

But that is when we look at the firing event as a whole, which is simplistic (if mostly correct) way of looking at it. In reality that bullet is not accelerated instantly but over some time. And in the same fashion the gun is not thrown back instantly, but over some time. If we look at the rate at which this momentum is generated N*s/s we get... tada! Force! And AllTheNothing is correct that there are limits to the amount of force a human can withstand, and increasing the velocity of a round also increases the necessary acceleration and thus the force (because as he said, F=M*A).

Lets evaluate a test sample to see how this might play out.

A conventional .45ACP round might mass 12g, and exit at 320m/s giving it a KE of ~600J. And a momentum of ~4N*s. Now if our .45 has a 12cm barrel it must accelerate at (at least) ~430km/s² or ~43,000Gs. And it will take it about .75ms to do it. Plugging this back into our force equation, we find out that the average force on the wielder is ~5.3kN. In reality the peak acceleration will be worse than this, as (unlike most railguns) a handgun does not accelerate it projectile evenly, but this figure will do for now (you can get tables on this stuff if you want).

Now let us assume we are designing a rail-gun that fires a .45 like round. Except it weighs 1/3 as much, and moves three times as fast. That is, 4g and moving at 960m/s (assuming same density and shape our projectile might be ~8mm/.3in). The momentum of this round will stay the same, ~4N*s, but the KE will be much greater ~1800J. If the length of our barrel (12cm) remains constant, then the round will have to accelerate faster to reach that higher velocity, or about ~3,800km/s² or ~390,000Gs (a heck of a lot, but a lump of lead can handle it). Taking .25ms (what do you know 3 times as fast, 1/3 the time!). Which results in 16kN, which is (ta-da!) 3 times as much.

Which proves a simple principle, if the momentum and acceleration distance are held constant, then the force of recoil is proportional to a bullets velocity.

However, while this above is correct, it is also simplistic as there are some very simple things gun designers can do to adjust these forces felt by the weilder. For example by installing a spring to lengthen the amount of time a weapon recoils, which is often part of the reloading action for firearms. As noted above the acceleration of firearms will very greatly within this, with peak acceleration often being 3 (or more) times greater then the average, due to the nature of their action. The smooth even recoil of a railgun may in someways be an advantage, especially when it comes to accuracy.

Actually, no. It isn't the equal and opposite reaction that causes recoil (though it does play a part) but the conservation of momentum that does so. The best way to understand the subtle difference is to grab your pants and pull them up as hard as you can, in an attempt to fly. This will fail miserably, due to the fact that while your arms are pulling up on your pants, your pants are also pulling down on your arms. And, more importantly, while your pants are pushing up on your torso, your torso is pushing down on them with equal force.

Within a closed system, all forces cancel each other out. So you can't move a box from inside that box and you can't fly by pulling up on your pants. Attempting to do so only leads to atomic wedgies.

Momentum is conserved. This is actually related to and intertwined with the Third Law of Motion and determining which came first in the grand scheme of the universe is very much a chicken and egg problem.

The firearm, powder, casing, and bullet are, for all intents and purposes, a closed system until shortly after the weapon is fired. The linear momentum of a closed system cannot be changed from within that system. In other words, the actual deflagratiion of the powder and acceleration the projectile does not cause any sort of recoil. If the barrel of a gun were perfectly sealed so that the bullet could not leave, you could accelerate the bullet up to light speed within that barrel of, because it is all part of a single system in which momentum is conserved.
Note that this fact does not prevent some parts of the gun from moving due to the forces exerted on them by the gases accelerating the bullet while they are still part of the same closed system, it just means that the gun's center of mass will remain stationary. Depending on how the force is applied, this could cause the gun to rotate in place, which the shooter could feel.

But the bullet leaves the barrel of the weapon, and so do the propellant gases. In doing so, they take momentum out of the system, out of the gun. In the shooter's frame of reference, the gun is stationary. It does not have any momentum. But that is alright, because momentum is a vector. It is possible an object to have negative momentum. Subtracting momentum from a stationary object causes that object to move backwards according to the equation p=mv. The sum of the momenta of the bullet, the gases, and the weapon will be zero.

One can calculate this fairly easily. And one can determine the force applied to the shooter be taking the derivative of the Impulse, which is equal to the change in momentum over time.

Incidentally, the conservation of momentum is also the single reason why you cannot knock the Earth out of its orbit by convincing everyone in a single hemisphere to jump at the same time.

The only thing I object to is the moving a box while inside it. You can do that. Other than that it has been 12 years since I've had physics so I'm just nodding along.

Yup, all the felt recoil comes after the projectile leaves the barrel. usually because (on an automatic) the bolt, slide, whatever delays the felt recoil until it hits the rear most point of movement, at which point it slams to a stop and all that momentum is transferred into the frame and from there into your hand or shoulder, keep in mind, however, that automatics are less efficient than other weapons because they use some of their energy to work the mechanism as opposed to putting all (or as much as possible) into the projectile. With revolvers, black powder, breech loaders, it's just because the bullet is moving so fast that it's gone before the weapon (being heavier) even starts to move, but, if they have the same propellant and bullet weight they'll recoil harder and sooner while putting more of their energy into the projectile.

The felt recoil can be reduced fairly easily, actually, just by increasing the weight of the weapon (preferably in the front of the weapon to reduce muzzle climb but overall weight increases help too) or allowing the barrel to recoil inside the frame of the weapon kind of like the Barret .50 does, also, it helps if the barrel, frame, and stock are aligned. One of the main reasons that the M-14 has such hard felt recoil is that the barrel and receiver are actually above the stock, thereby throwing off the alignment and putting the recoil line above the stock as well, several new drop in stocks are available that put it in line, reducing the felt recoil, and making it, once again, a battlefield favorite. Also, remember that a shooter will usually pivot from the hip when firing a powerful weapon throwing his upper body back while his lower body stays (mostly) in place, hence the SR hip-pad for the assault cannon and such which puts the weapon lower on the body and changes the way the recoil is perceived by the shooter.


What do you mean?

Atomic Wedgies? Isn't that a band?


I feel your pain. Me too, mostly.

You can tip a box while inside it, applying enough force to one edge that it rotates, and from there gravity takes over. You cannot, however, push a box from the inside. And if you're struck in a box in outer space, then you're SOL unless you have something to throw out of the box with great force. Anything else would require physical contact with something outside of the box, either directly or indirectly. If the box is sufficiently rigid, and is in the presence of gravity, and is on a sufficiently rigid surface with sufficient coefficients of friction, you might be able to make the box vibrate strongly enough for it to walk itself some distance.

I dunno, it seems to me that if I'm inside a cardboard box I can scoot it along by jerking forwards with my butt against the ground... Though I guess that wouldn't really be pushing the box. What you're talking about, if I understand, is like being inside a big box with your feet against the bottom and your hands against the wall, and pushing on the wall to make it move. Just like pulling up your pants won't lift you, pushing against a box won't push the box forward when you're inside the box, even though the same pushing from outside might move it easily.

See there is a difference in saying "..you can't move a box from the inside..." and "You cannot, however, push a box from the inside."

It is true that I cannot push a box from inside it, it is false that I cannot move the box from the inside.

Actually, yes, it is. The law of conservation of momentum states only that momentum does not change without the addition of an external force. "An object at rest/motion stays at rest/motion."

Meanwhile, every force vector has an opposite and equal negative force vector. So if an explosion exerts a force "down" the barrel, it exerts a similar force on the weapon "up" the barrel. A math example is below for those who care.







Actually the planet would move, however the centroid of the whole Eath-people-Moon system continues along the orbital plane. Gravity pulls the people back together and the Earth returns to it's original position. Now have everyone jump with enough force to escape earth orbit. You've just invented the mass-driver. The centroid of the Earth, Moon and stream of people does in fact maintain the same orbit but the planet will be shifted out of orbit.

That's an extension of the cardboard box example. Contemplate a frog in a cardboard box floating in zero gravity where mass frog >> mass box. The frog can jump to the other side of the box and from it's point of view, it's moving a lot. If you could see through the box you'd see the frog is mostly stationary while the box is doing the majority of the moving. From the outside, the box starts jerking side to side as the frog shoves the box away. However the actual momentum of the box is unchanged as the box-frog system maintains a stationary center of mass.

When you scoot a cardboard box along the ground you expand the system to include ground. Your scooting also relies on friction coefficients; if the ground was frictionless it wouldn't work.

depends on material of box and ground does it not?
if you have enough room to accellerate, run in direction of wall, jump for another boost, collide with wall and hope you do apply more force to the box than is neccessary to move it?
and in SR at least, it would so totally work due to magic . .

I think you are failing to consider that what you described is not an isolated system but just a part of a bigger one.

as long as you're not trying to move the whole universe, technically you allways have a bigger system around you as a point of reference . .
i ain't fit in my physics, but if there's no gravitation keeping the box still, and i manage to move it through inertial force from me hitting the box from the inside . . with the box being my frame of reference here . . would it not move? probably first in the opposite direction of my feet while running, kinda like those people walking on balls or barrels or something like that . . but if it's only a box with not much mass being weightless . . it means i can move it from within by applying force . .
and in the world of shadowrun, i just say magic again to screw over physics mom telling it to look the other way untill i am done if it does not like what it will have to witness . .

There will alway be than recoil unless you have than reactionless engine or field.

Too many sciencist inoge the idear that there might be no close system per say. Than refrifer willnot it is consider than close system, but it isnot than close system as it is apart of than larger world. That why I say than reactionless engine which doesnot throw out exhaust is possible.

The only problem is that all the inertial force you developed was gained from interacting with the box, so it was actually moving in the opposite direction you were trying to get it to move when you impact it again with the exact same amount of inertial force that you had. Net effect, center mass point of you and the box remains in the same location. Go back and read the frog in a light box example above. Your the frog in the light box in the case, with you actually barely moving while the box is tossed back and forth around you. Though your point of view of the situation inside the box will be totally different since you can't actually see what is happening outside.

I'm sorry, it sounds like you're saying something interesting, but I can't understand you... Maybe a spell check wouldn't hurt?

I think he is trying to say that in fact that newtons 2nd law is incorrect and that you can have some sort of reaction where momentum is not conserved.

This of course is at odds with the thinking of pretty much the entire scientific community. But that hasn't stopped many a crackpot from dreaming up various sorts of "reaction-less thrusters."

Frankly I think homer said it best "Lisa, in this house we obey the laws of thermodynamics!"

If you were in the space (in assence of gravity) into the box and you were to hit the box due innertial movement than you were moving to begin with, at the time of the impact part of your momentum would be tranfered to the box slowing you down and making the box move, but the total momentum of the system would be unchanged (for purely innertial movement); if you were to moveby your own deliberate action than the equation must be expressed as energy, by using muscles you turn chemical energy present in your body into kinetic energy, the act of moving a limb alone adds a certain ammount of momentum to the system even if there was no ammount before, but the overall ammount of energy of the system is unchanged, yes you can move it by punching/kicking it.

But when you hit the other end of the box the motion stops. Yes, the box "moved" but in essence you are rotating about the centroid. You are exerting energy with no net increase in momentum. Even if you pingpong around the box, the overall momentum remains the same. All you wind up doing is creating heat.

BTW, keep in mind that the Thunderstruck circumvents Smart Armor too, as Smart Armor is only activated by weapons with a DV of 10+.


In fact, it did - see ANDREWS in Rigger 3.

Are you trying to correct me or to complete my reply?
Anyway I'm studing to become an engineer so your sign makes me so evious.

I wonder if it's porpousfull (as the statement about halving all but smart armor was used in all the gauss weapons) or was a mistake.




What PPC standes for?

The fact that the Thunderstruck has a DV of 9 and thus evades Smart Armor? That' pretty much on purpose - in fact, it was most likely specifically engineered to do so by Ares. Most likely, Ares is running heavy advertisment featuring some heavy power armor infantry sporting thunderstrucks, hunting tanks.

The fact that it has the same rule as any other Gauss weapon? It is still correct - it doesn't halve Smart Armor - it simply doesn't trigger it.

They're strictly equivalent anyway.

As for Coil/Railguns, also consider the fact that there is no cartridge - the whole momentum is given to the projectile, thus making the weapon more efficient, and generating less recoil for the same projectile momentum as a standard firearm.

As a sidenote to the discuss concerning which weapons do or do not generate recoil, even lasers have recoil (but so microscopic it's not worth talking about). Missile launchers do not, however (because there is no real barrel, the projectile being self propelled.)

PPC = Particle Projector Cannon.
One of the Biggest Energy Weapons from BattleTech

Well if we consider fotons as particles laser weapons could be considered PPCs.

RL Gyrojet were almost recoilless due minimum momentum impressed at the firing act (the bullet accellerated due a mini rocket engine); they were dropped due the ungodly cost of the ammunitions.

with hoy superconducter canon in SR a gauss rilfe would very very effective as even a slightly magnetic round would only make contact via the Meissner effect with the air and almost all the power put though the coils would be transfered to the round or at least to kenetci energy.

No, a laser is not a PPC. And I believe the particle weapons in Shadowrun are more closely related to modern research into particle accelerators and particle beams (been on-going since the 50s).

Yeah, that's the way I recall it, too. PPCs in Battletech/Mechwarrior fired streams of protons, if I remember correctly.

You mean H⁺ iones?

What else would he mean? Is there some other sort of proton out there that I'm not aware of?

probably somehwere in some remote or incredible hard to find place science just did not look at yet for some reason *grins*
and you're worse then most of the actual battle-tech players i know, really going into the physics of those guns . . eh, runners ^^

Battle tech is far sillier than SR. Giant metal robots that have severe heat issues. Hmm, that's three seperate guidance systems you can use for terminal homing....

Who? Me?
Stahlseele you are a such troll (it's meant to be a compliment, hey you play trolls).
Yet I completely missed the meaning, can you translate?

yes, you, all of you ^^
and yes, i take that as a compliment, thank you very much ^^

discussions about physics is one of the most repeated theme around these parts, because even with frigging magic, people still try to compare the world of shadowrun to our world to figure out how stuff is supposed to work in the game.
and with the years of experience in the most diverse fields around here, you're more or less bound to think you have clicked a wrong link and landed in some board of higher education for those specific fields ^^
i basically meant, that there's probably somewhere a place where different protons can be found. science just has not found them yet.
it was a joke. i am le tired ^^

Sorry it was that at the highschool when the chemestry prof had the habit of refering H⁺ iones as free protons (even if H⁺ iones could have neutrons as well).
It's been a glimpse of youth.

Hopefully you don't go into consulting civil engineering. It sucks. It paid 75% what I made in the IT field starting out, it took me 7 years to get back to what I was making in 2000. I got my PE in December 2007 which gave me an office and made me too expensive to survive the next economic hard time. I left this past July and I was informed by former clients that in September the office responsible for 80% of my workload was shut down.

I work in IT again, and I'm making slightly more than I did as engineer.

If you are civvie, spend your 20s in a consulting firm and find a decent niche and then in your 30s find a job in the public sector that you can retire in.

I had to pull out a lot of math this time around because as a civvie, momentum never comes into play. If a building moves, the building has failed. Most of my work was hydraulics, the most dynamic wing of civil engineering, and even there you deal more with force, moments, and deflections than energy or momentum. Energy and power only shows up when sizing pumps and powered control systems.

That description just makes me drool.

- J.