# How would weapons and armour be affected if metal weight was negligible?

I'm in the middle of building a fantasy world with one core component – a reasonably rare magical metal that gets lighter the purer it is, without losing any of its mass. Alloys of this and steel are a magical equivalent of Damascus Steel – sharp, tough and resilient, with the added advantage of weighing less, but having a slightly greater mass than a steel weapon of the same size. How would this affect the weapons and armour of the setting?

So far, I've come to the conclusion that magical armour would be in high demand as lighter plate mail and chain mail equals somewhat better manoeuvrability. Where it comes to weapons, obviously as they are negating the weight weapons would be wielded differently.

I'd come to the same conclusions as many of those writing in the attached question, that oversized weaponry isn't much use, but that lighter weapons for the same mass would certainly give an advantage to things like hammers and axes in warfare. It should be noted that too pure a concentration and things start literally floating, so a purely magical sword would actually not be very useful at all.

My question is, am I right in my assumptions that weapons the same size with the same mass that were "lighter" would be more effective in battle? Specifically thinking of swords, axes, and arrowheads here.

Currently my idea is that the metal in question would impart a magical "upward" force equal to its concentration. 100% "unobtanium" would equal 100% upward thrust, akin to if gravity were acting on the item the exact opposite direction, with 50% equalling gravitational influence (as if you were using it in a weightless environment) and 25% making the item 50% lighter. The mass increase would be akin to that of a steel quantity 5% larger.

• You need to be clear about the difference between weight and mass. For example, if a bullet negated the effects of gravity, then all that would happen is that it wouldn't drop towards the ground as it travelled, an effect which is quite negligible already over short/medium distances. However, if you removed its mass then it would quickly lose speed due to air resistance, and if it hit the target then they wouldn't notice: it would exert no force on them, since force is a product of mass. So, please be clear about mass (intrinsic property) vs weight (downward pull caused by gravity). – Max Williams Oct 19 '15 at 10:00
• I thought I'd made that clear in the question - Mass is slightly (Not significantly) increased per "unit" of magical steel, but weight greatly decreased. – Miller86 Oct 19 '15 at 10:07
• Actually yes you did, sorry :) – Max Williams Oct 19 '15 at 10:11
• Just a clarification question: if you were to hold 100% "Unobtainium" in the air and let go, would it just stay suspended? – nukeforum Oct 19 '15 at 14:39
• @Miller86: So the Unobtainium actually has negative gravitational mass? I thought you were going to say pure Unobtainium would accelerate upwards exactly like a helium balloon, pushed upwards with a force = the weight of the air it displaces. Your AntiGravitonium is thus much more valuable than merely weightless, because you only need 50% concentration (when alloyed with an equal-density metal) to create a zero-weight object. i.e. 50% by inertial mass, not necessarily volume or moles (atom count). Also keep in mind that alloys can be more or less dense than the components (atom lattices).. – Peter Cordes Oct 20 '15 at 5:17

Well, a high mass armor with high unobtainium ratio would generate lift. This would allow soldiers with such armor to have reduced weight. While the inertia of the armor would restrict the practical weight that can be negated, your specially trained elite imperial stormtroopers in their shiny white armor would be able to scale walls with ease, cover large distances with long jumps, run over quicksand and maybe even water. This would make them pretty good assault troops.

Assassins Creed style parkour on roofs or trees would allow ambush and hit-and-run tactics that would be very hard for normal weight troops to counter. No matter how good their horses are, they can't follow your troops to the foliage or rooftops.

And of course being able to negate weight would allow muscle powered flight with fairly simple technology. The advantages from being able to fly should be obvious.

I'd assume this would result in a lift generating, fairly massive, plate armor on the upper torso with rest of the armor and weapons being weight reduced but otherwise normal.

• (anyone, correct me if I'm wrong) @sumelic 50% concentration would negate the weight of the item, making the item weightless. Anything "purer" than that would change gravity in the other direction. So a 25 pound plate of 100% purity would pull up 25 pounds - making someone who weighed 100 pounds now weigh 75. Add a few more plates, and all of a sudden your 100 pound ninja, with all the muscle to push that mass, effectively weighs 5 pounds. – WernerCD Oct 19 '15 at 13:59
• I think there might be a big problem with this. Let's say I weighs 200 pounds. I put on 195 pounds of 100% armor,I now weigh 5 pounds. I jump as this answer suggests either up very high, or over long distances. But I have 400 pounds of inertia. As soon as I land, I get crushed and die. – Shane Oct 19 '15 at 17:29
• @Shane Yes, as Michael says you are still not getting free energy to kill yourself with. But it definitely will create the need for the "special training" I mentioned in the answer since untrained people would have issues stopping or changing direction. Or using their weapons effectively with something constantly pulling their upper body upwards. But I figured covering all that would be beyond this question. – Ville Niemi Oct 19 '15 at 21:12
• @Shane Ya, no, that isn't how it works. Do the free body diagram. If you weigh 50 kg, and you are wearing a -45 kg weight 45 kg mass item, you fall 19x slower. Your inertia is that of a 95 kg being, but your weight is that of a 5 kg being. You only fall at 0.51 m/s^2. The only time that mass/weight differs like this in our experience is with buoyant balloons and the like. So your physics intuition is leading you astray. – Yakk Oct 20 '15 at 17:29
• @Shane Think of it as being in normal armor with a small blimp negating most of your weight attached to a shoulder harness. Except for having no drag that is how it should work out. – Ville Niemi Oct 20 '15 at 21:18

Removing the weight but retaining the mass means that the inertia is also retained. This has several consequences.

Imagine if someone were to try to run and then subsequently trip while wearing a full plate suit of this metal. They would end up tumbling due to the mass of the suit, but only slowing down due to the friction caused by the weight of everything else. As such there wouldn't actually be much mobility improvement, they would simply get less tired from standing or marching in it. "Lighter" suits and pieces would scale similarly, with perhaps the least massive ring pieces offering some mobility advantage.

Melee weapons would suffer similarly; holding one would be less tiring, but swinging one would require just as much effort.

The main application of such a material would probably be missile weapons. Projectiles, due to their lack of weight, would fly truer and much farther. Armor-piercing ballista bolts as heavy as a toothpick would be a strong defense for any city above the size of a village... or in the siege weapons against such a city. Even crossbows would see some benefit from lighter quarrels, as would arrows made almost entirely of the metal.

• I think that melee weapons would be a bit easier to swing, since some of the effort required for the upward swing is counteracting gravity, which would be removed. On the flip side, the force of a downward impact would be reduced for the same reasons. – Max Williams Oct 19 '15 at 10:13
• @MaxWilliams: You can compensate for the reduced impact by increasing the mass, but then the upward motion and subsequent stoppage before the downward stroke eliminates any weightlessness benefit. – Ignacio Vazquez-Abrams Oct 19 '15 at 10:15
• "They would end up tumbling due to the mass of the suit, but only slowing down due to the friction caused by the weight of everything else." So that's what's going on in GTA! – overactor Oct 19 '15 at 11:06
• Ballista/crossbows would only really benefit from this in accuracy and distance, and might actually lose a little power (e.g. shooting a crossbow down from a wall). – JonTheMon Oct 19 '15 at 15:25
• I don't think it's likely that much of this precious stuff would be used for projectiles at all. – leftaroundabout Oct 19 '15 at 22:02

For armor:

You wouldn't have armor that is similar to our world's middle ages armor, but lighter and providing more maneuverability. Instead, you would get massive armor that weighs the same as our world's middle ages armor did. Knights were the ancient Main Battle Tank. And look at today's tanks. We don't reduce the thickness when we come up with better, stronger alloys. If we come up with with a lighter alloy, we don't get faster more maneuverable MBTs. We thicken up the plating.

Instead of chainmail light as a tshirt, your unobtanium would create massive nearly invincible behemoths.

For weapons:

I don't think swords or axes would be more effective in combat. The force of a blow is mass times speed. The mass of these weapons is the same. And the speed they are swung at won't change either. At least at first. The biggest advantage would be that you can swing your weapon all day without getting tired (assuming you survive all day). So your first blows in a day will be the same strength as a regular weapon. Later in the evening you might be still hitting just as hard, where as with a steel weapon you would be swinging slowly because you are tired. On the other hand, running around all day fighting for your life has got to be tiresome all on its own. I think that would overwhelm any advantage in energy the lighter weapon would provide.

Bows and arrows work out to the same, but worse. The limiting factor is how hard you can pull on the bow. The arrow being lighter doesn't change that. You would get just as tired pulling on that bow regardless of what arrow head you used. The only good thing with the lighter arrow is that it would have longer range. This may or may not be useful depending on the situation.

tl;dr: Your weapons will be hitting just as hard as regular weapons, but they'll be hitting people wearing armour a foot thick.

EDIT: You might be able to use this to enhance the effectiveness of pike or spears. If you balanced the weight well -- having a 'lift' force counteracting the weight pulling down -- you could possibly effectively use stupidly long spears.

• Swinging a sword around is hard work whether gravity is pulling on it or not. Most of the work your muscles do goes into accelerating the sword, which then hits something and stops. It's only if you're just carrying the sword around all day that the gravitational weight becomes a big deal. Your conclusion is correct though: heavy armor, regular weapons. – Peter Cordes Oct 20 '15 at 5:22
• Sure, but swinging around a couple of pounds makes you more tired more quickly than swinging around a couple of ounces. Try it. Swing your arm as hard and fast as you can. Then do it again with a 5 pound weight. I assure you, the weight will make you more tired. :) – Shane Oct 20 '15 at 14:05
• you're missing the point. The effort required to swing something around depends on the inertial mass, not the weight. I unfortunately don't have any matter available which has different inertial and gravitational masses. If I did, I'd win a Nobel prize for discovering it, and disproving Einstein's theory of relativity. (acceleration is indistinguishable from gravity). – Peter Cordes Oct 20 '15 at 21:11
• @shane the difference you're missing is swinging 5 pounds in the air, or swinging 5 points across a table on a frictionless coaster. It still has the same mass, you just don't have to counter-act gravity – JonTheMon Oct 21 '15 at 15:12

# Initial failure

You could not simply replace standard gear with this new gear and expect a positive outcome. Unless they have extensive training with the new gear, the first battle in which troops are equipped with this gravity free equipment you will make two major observations:

Troops arrive to the battle fresh and feeling ready for combat.
With reduced weight, there should be less effort spent to walk in straight lines.

The troops using the new gear will lose dramatically.
Due to the decreased weight, they will feel more maneuverable than they actually are and either be slow to react, or constantly throw themselves off balance. Both are costly in a life or death situation. Furthermore, if the armour is actually more massive than their typical gear, you can expect a lot of torn ACLs when they try to stop or change direction due to the increased horizontal momentum.

When using the weapons, they will not be able to hit a target (literally to save their lives.)
All the muscle memory built training with normal weighted weapons includes a component to counteract gravity, without gravity most swings and jabs of melee weapons will be high.
Any upward swing which misses its target risks dislocating the wielder's shoulder as the swing will reach its apex without losing any momentum to the pull of gravity.

Targeted projectile shots will go high of their mark, how high depending on range.
Volleys will fly long. If these shots do come down at all, it might not be point first. This would depend on if just the arrow head, or the entire arrow is weightless, and whether the head or the fletching provided more wind resistance.

# Indefensible Siege Weaponry

Over time you will see changes to take advantage of the weightlessness, but this might not be on the level of the individual soldier, I see this as a means of making indefensible siege weaponry:

Friction-less Battering Ram
Picture a truly massive battering ram of mostly weightless material. Start pulling it with teams of horses toward your target, it might take a few miles (in a straight line) to get up to speed, but you could release it even out of range of sight of the target and it would glide in with its unstoppable momentum.

(near) Infinite Range Catapults/Cannons
If a 50% blend of the new metal makes perfect weightlessness, with careful blending, ballistics could be made to fire at nearly any range. The only limitation being wind resistance.

• Concerning your last point, you wouldn't need to re-blend the material for each shot. Rather, have a cavity in the shot where you add (or don't add) various amounts of weighted material. That allows your siege engineers to vary a shot's weight in the field, with only a few moments notice. – Martin Carney Oct 19 '15 at 21:42
• The battering ram is really interesting. Though I doubt releasing it far from the target would be clever, because it would be hard to aim accurately enough. (The best implementation might actually be to build the ram as a ridiculously massive tubular armour for a horse, and ride it right through the gate!) For the same reason, I don't think the infinite-range catapults make much sense: it's little use to reach over many miles, if most of the shots only hit trees and meadows. – leftaroundabout Oct 19 '15 at 22:27
• Having the inertial centre of mass != gravitational centre of mass would cause a tendency for arrows to pitch up, but fletching should be more than a match for the task of keeping the arrow oriented correctly. An arrow going slow enough to tumble wouldn't do much damage even if it did strike point first (assuming we're not talking about super-dense arrows that have way more mass than the fletching is adequate for. This AntiGravitonium isn't super-dense in the inertial-mass sense.) +1 for everything else though, great points. – Peter Cordes Oct 20 '15 at 5:34
• This stuff is worth more than gold and you want to make projectiles from it? – Stig Hemmer Oct 20 '15 at 10:34
• But then... they get the stuff! – marcellothearcane Jul 25 '17 at 20:47

Considering that the potential energy scales with weight (weight * g * height) but the kinetic energy scales with mass (mass * velocity^2 / 2), you either

a) don't have conservation of energy, hence a weapon could utilize a perpetuum mobile for infinite energy, or

b) in order to retain conservation of energy in that universe, things with less weight not only need to be less heavy but also to fall slower, so gravity would not accelerate everything at the same rate anymore.

This last point might be an interesting thing to consider for (simple/middle-age/steam-punk?) technologies that might be established in that universe.

• Currently my idea is that the metal in question would impart a magical "upward" force equal to its concentration. 100% "unobtanium" would equal 100% upward thrust, akin to if gravity were the exact opposite direction, with 50% equalling gravitational influence (as if you were using it in a weightless environment) and 25% making the item 50% lighter. (this point now added to the question) – Miller86 Oct 19 '15 at 12:17
• Hmm, according to link the interaction between positive and negative mass would have the positive mass (earth) still attract the negative mass (unobtanium) and the negative mass repel the positive mass (so the unobtanium would fall to the ground and the two would form a run-away system moving through space), but since this does not fit your design I am not sure if there is a scientific concept that would allow for the behaviour you described. Hence it's hard to discuss the physical implications of what would happen in such a universe. – Marius Oct 19 '15 at 12:59

First thought I had was that giant tower shields would be back in style. Yes you can make bolts that are "lighter", but the stresses and strains on the shooting devices wouldn't change. Instead, you can lug around giant shields, that yes, they have a lot of momentum, but you can just push it around as you need to and it'll protect you from most anything.

One aspect of this metal would effect architecture. There was a lot of work in the Middle Ages to make taller buildings using relatively light weight building materials (cathedral architecture). With a metal that can be made pure enough to literally resist gravity, you could do interesting things with super tall towers made of a skeleton framework of the stuff, or even a platform of uber-pure metal that floats, held to the ground with a tether, that you can perch archers on. They get an arrow proof metal shield under their feet and they can hover right over an enemy force and hit them from overhead at will. Have a couple floating platforms tethered to the corners of your castle for defense, or use them to get fighting troops up onto an enemy wall as an attacker. Lots of possibilities.

How about a lighter than air throne held down with a hook/lever to the stone floor. If they breach the throne room, the lord pulls the lever and floats up and out through a hole in the roof.

What about a "reverse catapult" that uses a block of unobtanium as a "counterweight" like a trebuchet? Instead of a heavy block to pull down on the end of an arm, it's a light block that pushes up. Fiddle around with the design and some trebuchet type thing would be feasible. Why would you want this? Because the weight of the counterweight made a typical trebuchet hard to move. You have more mobile artillery. Maybe even to the point where artillery becomes a factor in field battles more like the Napoleonic era than the middle ages.

(This answer is all about 50% alloy weapons and armor, since I think you're only asking about these.) At first, it didn't seem that useful to me. I have no experience using real-life weapons, but I would imagine that melee weapon techniques have been developed to take advantage of gravity (for example, on a downstroke, the force of gravity is added to the force imparted by the person holding a sword, making a more powerful blow). Therefore, taking gravity out of the picture doesn't necessarily make things any easier (this was pointed out in the comments). It also requires the person using the weapon to use different techniques from regular sword-fighting, so they either have to re-train or train from the start with special, expensive weapons.

However, since the material is rare, it's possible that opponents would not have much experience fighting it. So for melee weapons, the biggest advantage might be the unexpected behavior compared to regular weapons.

I'm not sure about armor yet. Weighing less could reduce fatigue fr carrying heavy armor. Of course, the inertia is the same, but with your armor the person weighs less, which means all else equal they'll have less friction with the floor. So it seems to me that they may have less traction (a possible disadvantage). But I don't really know. One advantage seems to be that they would be able to jump higher.

• 50% would make things easier, from the get go. Full Plate Armor of 50% purity would be weightless. You weight 150 pounds? Put on a full suit of 50% and... you still weigh 150 pounds. Assuming it stops stuff the same as Plate (I'd assume magic armor would stop stuff better... especially 100% pure) Create a backpack that weighs 25 pounds of 100% purity (say, some sort of chain mesh... or magically created 100% pure "thread") and put 30 pounds of stuff in it... it now weighs 5 pounds equivalent. – WernerCD Oct 19 '15 at 15:43

I'm surprised no-one's mentioned aerial warfare: with the stuff as described in the question, you could use it to fly high in the air.

Of course, controlling altitude and velocity would be a major challenge, and the only way to come back down is to let go of some very expensive antigravity ballast, but still, it might be useful from time to time, for surveillance or for dropping bombs of some kind.

So pretty much what I'm getting from the majority of these questions is that everyone will be wearing foot thick armor. Hand-held weapons such as swords, axes, etc. would be placed at a significant disadvantage due to this fact. Maneuverability of the soldiers would be slightly hampered due to the bulky nature of their armor, and there would most likely be some weak areas of the armor that the attacker can use to their advantage.

So, you might ask, How can we solve this? How can anyone be killed?

Well, since we can now apply much more metal to our weapons, since we seem to have an unlimited supply of metal and unobtanium, we can start to delve into huge steel catapult balls, ballistas, etc.

Kitting up animals with unobtanium armor would be extremely effective. Let's say you train up either a group of dogs or another such animal that's pretty sturdy and can run relatively fast. Kit those bad boys up with some unobtanium armor and set them loose on the enemy. Make sure to pad the armor up pretty heavily, but then, they'll be pretty much impervious to the enemy as long as they keep running out of harm's way after they ram someone. Rhinos would be extremely effective, too. If you kitted a few dozen rhinos with unobtanium armor, a ridiculous amount, due to their immense strength, and set them loose upon the enemy, they would be incredibly effective. Especially if they ran down a hill. You could simply kill them off by injecting a toxin in their system that would allow them to live for five minutes longer, and hope that they would die before they got to attacking your forces. Or, you could hide an explosive in their armor that would essentially turn them into a grenade, if you're really that intent upon winning.

You could also create giant contraptions with this newfound metal, because wheels would be able to carry more of it. Also, you could make a metallic model of Da Vinci's tank a few hundred years before tanks became a thing. Da Vinci's tank was a revolutionary idea, and if his ideas were copied off by someone, they could probably have been used to create the first tanks almost 400 years before they were in our timeline. It's hard to say how effective and thick the armor on Da Vinci's tank would've been in your timeline, but nevertheless, it would've revolutionized the way people thought of warfare, and new inventions and ideas by people who in our timeline never got to think/work on such things would be available. Who knows, maybe WWII tanks would be commonplace a hundred years before WWI started.

First, let's go over what it WON'T do. It won't help ground troops. Reduced weight only helps bring things up, but would actually make moving things horizontally much more difficult. The reason is that the inertial mass is the same, requiring the same amount of energy to gain speed, but the friction with the ground, which is the main method in which ground troops propel themselves, is actually reduced. It's like trying to move around in reduced gravity, or on ice.

So, option one would be to replace friction-based propulsion with other methods, e.g. thrusters. But that doesn't sound like the type of setting you're going for. It would essentially result in hovercraft and aircraft.

However, a much more interesting use would be kinetic bombardment. With friction removed, it would be possible to slowly build up a large amount kinetic energy in a sufficiently large quantity of unobtainium. Imagine a large floating boulder, neither moving up or falling, being pushed across a large distance. Initially, it's very difficult to move, because it's very heavy. But with no friction slowing it down, it would just keep gaining more and more speed. Then, you just let it keep on flying until it hits an enemy fortress or something. The sheer amount of kinetic energy it gained over potentially hours of pushing would be incredibly difficult to negate. Air drag would still be a factor, but against a sufficiently heavy and/or aerodynamic object, not a significant one.