View Full Version : science question for fantasy novel:

Flawed Creation
10-12-2004, 08:56 AM
If a sword fell from around cloud height and hit a boulder-sized rock in the middle fo a forest, what would happen?

how far away would this be noticeable, either because of sound or vision?

what condition would the area where it landed be in?

this bears on the beginning of my book.

10-12-2004, 09:06 AM
How much does the sword weigh?

Which cloud height are you talking about? Different clouds live at different heights.

What kind of boulder? Granite? Limestone?

Flawed Creation
10-12-2004, 09:39 AM
well, i didn't expect an easy answer. if someone knows the best way to leanr this stuff...

the variables can be adjusted as neccessary.

the result i'd like is- the sword survives, the angels on a nearby cloud notice this event, come to investigate, and the hero draws the sword form the stone. i don't need the stone to be truly intact, but i'd prefer something more than a pile of dust. some incidental damage is acceptable, but this isn't noticed by the world at large. luckily, the forest is on the edge of civlization.

the weight of the sword: I haven't entirely decided what type of sword it is. it will be used in two hands, but it's definitely not a claymore or a katana, the only types of two-handed swords that immediately come to mind. it's definitely sharp, unlike a claymore, and is light enough to be moved relatively quickly and skillfully.

of course, the wielder is an angel who is 7 feet tall and is very strong by human standards, so you never know.

I haven't read up on clouds yet- the angels use enchanted clouds for transportation, and the major characters are keeping a watch from the clouds. the magic clouds are intended to blend in with real clouds though. the should be close enough to the ground to see what goes on, but they have vision like hakws so that can be fairly high.

the sword which is dropped can be any height sufficient so that they would notice it, either in falling (resembling a shooting star? especially if it magically glows?) or on impact (explosion? clanging sound? sonic boom? [if magically accelerated])

the sword is being dropped by a villain who wants them to notice and investigate, and in particular to take the sword for themselves.

the boulder can be of any convenient stone that would be found in a european forest. i have the humans speaking a pidgin-german (or possbily something scandinavian- i haven't worked much with the humans yet) and i figured a german climate would make sense.

10-12-2004, 11:25 AM

Let's say the angels ride above strato-culumus clouds. (http://www.enchantedlearning.com/subjects/astronomy/planets/earth/clouds/) That gives you a height of 6,500 feet, mebbe.

Let's say the weight of the sword (http://www.brainyencyclopedia.com/encyclopedia/l/lo/long_sword.html) is 4 lbs, just to be on the large side.

If you have an Earth-like gravity, the sword will fall at 32 ft per second per second (to simplify). According to my rough calculations, it will take 8 seconds to fall that far, and it will hit the boulder at 2,048 feet per second.

Ouch. Let's say aerodynamics encourages the sword to fall point downward. The tip of that sword will strike with

KE=1/2M * V(squared) (http://www.glenbrook.k12.il.us/gbssci/phys/Class/energy/u5l1c.html)

So the kinetic energy would be half four lbs times 2,048 squared. Or 8,388,608 lbs/fpss

Let's pretend you wanted to compare that to a two thousand lb car. By my (probably flawed) calculations, that car would have to be driving just over 91.6 feet per second when it hit to have an equal force. That's about 62 mph.

Let's pretend you took a lot of useless mass from the back of hte car, the roof, the radio, whatever, and built a sword-like spar on the front of it, along with the structure that could hold the car's frame solid. Then let's pretend you rammed that spar by remote control into a rock at 62.5 whatever mph.

How would that look? How would that sound?

Not the same, I'm sure. But it could be close.

I'm not that good at this stuff. Maybe someone can step in and correct me.

10-12-2004, 11:40 AM

I'm not about to analyze your calcs, HConn; they look good enough. I think your judgement of aerodynamics of the sword is correct; the air resistance of the hilt would cause it to follow the point. But you have 4 tons of kinetic energy applied by the point of the sword (negligible area) to the rock.

It seems to me there'd be a rather LOUD thunk, followed by virtual anihilation of the rock, followed by the sword buried up to, if not beyond, its hilt in the soil beneath the rock. I doubt there would be any 'clanging' of metal, as you might expect, if say, the sword were to be swung into the rock by the 7'-tall angel.

Flawed Creation
10-12-2004, 11:12 PM
well, I suppose i don't need the sword to be in the stone. it was only for mythical allusions.

the sword sinking into the ground is fine with me.

how far away would the thunk be audible? (this si one of the more important details.)

what condidition would the area around the impact be in? would there be a shockwave?

10-12-2004, 11:38 PM
Wow, HConn....

:: walks away muttering something incoherent about abismal physics classes ::

10-13-2004, 01:00 AM
how far away would the thunk be audible?Based on HConn's physics, which I have no quarrel with, I'd think you'd hear the 'thunk' about as far away as you'd hear a car crash--maybe a quarter to a half mile--unless it happened in an acoustically enhanced area, such as a canyon or a vast empty plain. Such things as trees, grasses, or other 'soft' and irregular items will absorb the sound, diminishing the distance it travels. If your sword fell into a forest, I doubt the sound of its crash into the rock would be heard more than a few hundred yards away.

As to the condition of the surrounding area after the crash, again it would depend on what was there. In a forest, ree trunks would be scarred, with bits of the rock imbedded in the bark. On a rocky plain, there would be a small crater (small, being not more than a meter in diameter), with pieces of the destroyed rock scattered as far as a hundred meters away. Maybe. We could ask HConn to work out the physics, but I think it's not that important. The sword isn't massive enough or travelling fast enough to cause the kind of devastation that a meteor would.

10-13-2004, 07:23 AM
Pitching in as a not-a-scientist (I write fantasy rather than SF) all I can add to that is that, if the people hearing are angels, you could assume that their hearing is more acute than that of humans, so I don't think the carrying-distance of the sound would be a major problem.

10-13-2004, 10:15 AM
Flawed, there are so many variables for the sound you shouldn't worry about it.

Also, I'd have your character pull the sword out of a shattered oak tree.

Maca, I had terrible high school physics classes. Guys were doing bong hits in the back of the room while the teacher nattered on, oblivious. Google did all my thinking for me.

BTW, on another thread I recommended that everyone writing sf/f subscribe to SFWA's Bulletin. (http://www.sfwa.org/Bulletin/) Every issue, Robert Metzger writes a State of the Art column that does a grownup version of what I did. The latest column was about gadgets. Previous columns were about melting the polar ice to create "Waterworld" or creating a system where humans could draw all of their calories from sunlight.

Cool, cool stuff.

10-13-2004, 05:35 PM
Guys were doing bong hits in the back of the room while the teacher nattered on oblivious.

:lol That sounds really familiar, HConn.

10-13-2004, 08:58 PM
Sounds like a less dangerous version of the guy in my science class who one fine day opened up the gas valve for the bunsen burners and lit it so that a jet of flame shot across the aisle.


It's amazing he didn't blow the top floor off our high school and us with it.

10-13-2004, 09:41 PM
We had those gas jet nozzles, too, but the administration was smart enough to cut off the gas. Otherwise, we would have been closing the class with gas scares every period.

We did have a guy who set fire to his own shirt, though. On purpose. He was bored and laying a lighter flame on the sleeve of his flannel shirt, then beating it out. After about a dozen times, one of the fires got out of control.

10-13-2004, 11:45 PM
In high school chemistry lab, after a demonstration on how elemental sodium reacts with water, one kid stole the lump of sodium (about the size of a tennis ball), took it still wrapped in its oily paper toweling to the restrooms and flushed it.

About a half-hour later, they shut down the school to repair the plumbing.

10-14-2004, 12:50 AM
Sounds like a less dangerous version of the guy in my science class who one fine day opened up the gas valve for the bunsen burners and lit it so that a jet of flame shot across the aisle.

Funny you should say that. I just got back from chem lab about 15 minutes ago and did that exact same thing myself. Whoops.

You'd think us college kids would learn. :smack

PS - Chunk, what is it with me and fire??

10-14-2004, 06:07 AM
Generally speaking, a sword that falls from the clouds will be falling no faster than a sword that falls from 100 feet. Atmospheric resistance stops an object from accelerating once terminal velocity has been reached. For most objects, terminal velocity is only 220 miles per hour.

If the planet had no atmosphere, the velocity would increase of a per second per sceond basis until the planet stopped its fall, but it simply doesn't work this way on any planet with an atmosphere.

It's like a bullet. Fire a bullet straight up and it may leave the barrel at 4,000 feet per second and reach an altitude of four miles. But when it falls back down it will be traveling only as fast as atmospheric resistance will let it.

So your sword falling from the clouds will only be traveling about 220 mph when it hits, no matter how much it weighs. (Assuming it isn't a mile long.) The atmosphere slows metal down, too. A meteor weigh a thousand times as much as any sword may enter the atmosphere at 14,000 mph, but by the time it reachs the ground it's seldom traveling faster than 300 mph.

So even if your sword was thrown from Mars, it still isn't going to be going very fast when it hits the ground.

10-14-2004, 06:18 AM
I couldn't find any information on terminal velocity for something as heavy as a sword, so I ignored it.

But I'm sure James is right.

10-14-2004, 06:51 AM
Terminal velocity affects anything of any weight. It will, of course, affect a feather far more than a lump of steel, but even metal slows trememendously. An aerodynamic shape lessens the stopping power of the atmosphere, but not by a very great deal. Even if the sword falls point down, it will still max out at under 500mph, under the best of circumstances, and probably won't pass 300.

Aerodynamics will help an object retain velocity, but a falling sword starts out at zero velocity, so there's nothing to maintain.

Swords are heavy. Most who hold one for the first time can't believe how much some weigh. They range from three pounds to more than twenty pounds.

But if it weren't for the ability of the atmosphere to drastically slow down how fast a heavy object falls, we wouldn't need to build bombs. A 200 pound chunk of metal falling at 32 feet per second per second from 20 miles would have more explosive force than a bomb.

Kida Adelyn
10-17-2004, 04:50 AM
wouldn't, say a granite rock, react differently than a softer stone such as sand stone. :shrug I don't know anything about physics past basic motion physics.

I think having it fall into a tree would give it less of a King Aurthur kind of feel (unless that's what your going for) and since it's angels your dealing with sound definitely does not matter.

Flawed Creation
10-17-2004, 06:47 AM
yes, i *am* going for a King Arthur feel.
more specifically, i'm pointing out the absurdity of the King Arthur myth. the belief of the person who finds it that it's a sign from heaven that he is the savior is misguided.

i don't belive in chosen kings, and i don't think magic swords have anything to do with it.

10-17-2004, 01:10 PM
But if it weren't for the ability of the atmosphere to drastically slow down how fast a heavy object falls, we wouldn't need to build bombs. A 200 pound chunk of metal falling at 32 feet per second per second from 20 miles would have more explosive force than a bomb. A good example of using "thrown" objects as a weapon is The Moon is a Harsh Mistress by R. A. Heinlein. In it, rocks were lobbed at Earth from the Moon, and had more devasting effects than nuclear weapons. Of course, there, it was an issue of mass coupled with velocity. If your angel threw the sword to the ground instead of just dropping it, there is potential for greater destruction upon impact. But James' point about terminal velocity is something to consider.

However, for the purposes of the story, have the sword do what you want it to. If you need major destruction, have your angel hurl the sword to the ground. Only the true pedants will worry that you've violated any laws of physics.

10-18-2004, 07:28 AM
There's a widespread theory that the sword in the stone actually derives from a Bronze Age ritual where the young prince pulled his sword out of the stone mould it was cast in. Which would mean that it was his status that caused the ritual, not the other way round.

10-18-2004, 10:06 PM
That's really interesting. Thanks, Nyki.

10-19-2004, 07:28 PM
Oooh, Nyki... :jump

Tidbits like that put my brain into overdrive. Thanks.

10-20-2004, 01:21 PM
Mass is an important consideration, not so much because of velocity as because of inertia. Would you rather be struck by a ping pong ball traveling 300 mph, or a car traveling 50mph? Of course, mass and velocity together make for a wicked combination. But the faster something strikes the atmosphere, the more impact the atmosphere has on it. The atnmosphere works much like water. If you dive into the water from ten feet, the impact isn't much. Hit the water traveling 220mph, however, and it's like slamming into concrete. Air works pretty much the same way.

But if something has enough mass, it doesn't have to strike the ground very fast to release a lot of energy. Inertia does the job just fine.

But I wouldn't take the King Arthur myth too seriously. It's a parable, and contains some extrmely valuable lessons. Mostly, it's just a very good story with some very good characters.

And, of course, the Bible says God chooses all leaders, the good, the bad, the wicked, the foolish, all for His own purpose. So Biblically speaking, all kings and all leaders are chosen by God.

Be that as it may, true, false, of indifferent, if I find a stone buried in rock, and if no one else can move it, and I somehow manage it, trust me, I will think something special is going on. I'm not dumb enough to think I'm stronger than a bunch of young men with Conan biceps. That was the thing with King Arthur. It wasn't that he could pull the sword from the stone, it was that no one else could, including grown many with several times his strength. That sort of blows the mold theory, too.

But I wouldn't try faking the science of terminal velocity. Even most high school kids will bust you on this one. It's too basic. For that matter, neglect atmospheric resistance, and I doubt there's an editor in the world you could get the manuscript past.

10-21-2004, 07:27 AM
That would probably be true if it were a SF novel, but I seriously doubt that anyone would be thinking about atmospheric resistance reading a fantasy story.

Flawed Creation
10-21-2004, 07:34 AM
Nyki- I have no intention of violating the laws of physics except in those cases whre magic is explicitly involved, i know what the magic does, and i can't make it work within the laws of physics.

Pthom- the Moon is a harsh mistress is such a great book!

james A- I have no intention of violating the laws of physics. that's why i've asked this question. and I never said that no one oculd retrieve it from the stone but one person. actually, anyone could, but only one person tries. of course, not everyone would dare to, because it does spit fire and lighting when you touch it...

10-21-2004, 09:11 PM
There's lots of room to deviate from hard science in a fantasy (even in a non-hard-science sci-fi). As long as it's not too outlandish, I think you will be fine. No readers are going to stop and think, wait a minute...

10-22-2004, 07:51 AM
There isn't room in a fantasy for such a basic and obvious error in science. Readers absolutely will stop and see the gaff. You may as well have gravity not work. And an editor will certainly catch it. Make this an important part of the plot, and the novel is doomed. Not all readers will catch it, but a very high percentage will. It isn't like this is a new mistake. Writers make it everyday, and editors catch it everyday, and when it does sneak thrugh, readers laugh at it.

In other words, it's about as outlandish a mistake as a writer can make.

The moment you assume teh reader is dumb, you're through. The minute you intentiaonally allow a mistake because you assume it won't be caught, you're in serious trouble. It's a dumb, silly thing to do. Especially in fantasy.

Science fiction and fantasy share a great many of the same readers and the same editors, and making amistake like this will not only be caught, it tells the editors and the readers that you either think they're too stupid to notice, and you're to stupid to know it's an error.

It's too simple, too obvious, and too common to slip through, and why on earth would a writer knowingly make himself look silly by intentionally leave such a silly and obvious mistake that wouldn't pass even a casual reader who knows anything at all about extremely basic science?

Flawed Creation
10-24-2004, 10:24 AM

I couldn't agree with you more. I have no intention o faking science. in fantasy, it has always been myintention to scrupulously obey the laws f physice except when magic is involved.

in fact, even with magic involved i inted to keep physics as a guideline.

11-13-2004, 10:58 AM
I think it was Galileo that did the original experiment. I'm pulling this out of the deep dark recesses of my memory (or somewhere further down :) ), but I seem to recall that Galileo dropped metal balls from the leaning tower of Pisa. The balls were all of the same size but different weights, and some were hollow and others were solid. They all accelerated at 9.8 m/s squared. Mass has nothing to do with acceleration due to gravity.

A sword is balanced in such a way that it's extremely unlikely that it would fall point down - the centre of gravity is closer to the pommel than the tip. Aerodynamic flow would be extremely unstable and it would tumble erratically all over the place. That's why darts and arrows have most of the weight toward the tip and vanes on the back. I suppose your sword could have a very elaborate feathered pommel, and perhaps the tip could be weighted with the much vaunted legendary alloy Unobtainium.

The unstable (ie tumbling) descent would make for an erratic terminal velocity, but it would be relatively low as pointed out previously.

When the sword hit the rock it would clang and, if you were lucky, leave a small chip in the granite. If the rock was soft and compressible (such as pumice or talc), the sword was pointed at it, and the angular momentum (rotation) of the sword was close to zero then it might penetrate, but then an anemic toddler could pull it out with one hand, and I expect that somewhat defeats the purpose of the premise.

You could of course have the weapon hurled at the earth by the Angel of Oh God, I'm SO Sick Of Magic Swords, throw in some particle effects, perhaps one of those ludicrous planar explosions and a prophecy and you're away :) You could even have the wheels of a cart squeal on gravel, then have the horses fall over a cliff and explode. (Just kidding)

But seriously, all you have to do is Use Liberally Capitalised descriptions containing the words Holy, Magic, Avenging, Vorpal etc., or just have it Thrown Really Hard by a Seraphic Tart :)

11-13-2004, 03:45 PM
Angel of Oh God, I'm SO Sick Of Magic Swords:rofl

11-13-2004, 11:40 PM
I second Pthom's :rofl

Flawed Creation
11-17-2004, 07:34 AM
thanks, aramas. It doesn't really matter to me whether it's hard to puul out. in fact, having it stuck in a rock was not terribly critical, just an idea. but the part that makes it hard to remove the stone has more to do with it's tendency to incinerate those foolish enough to touch it than physical difficulties.

as for the physics, I might call on magic to help in some way, such as applying additional telekinetic force, but i still wanted it to be believable. using a soft stone is a good idea as well.

finally, with regard to unobtanium, it strikes me as incredibly unwise to make a sword out of it. as we all know, unobtanium is 4-dimensional and primarily used in the construction of klein bottles.

11-20-2004, 03:56 AM
There is an urban legend (well something i couldn't prove) that the force of impact from a penny dropped off of the empire state building is enough to bury the penny in the concrete below.

Talking about simple physics the assumption that the terminal velocity of the sword is that of a human is flawed. The aredynamic resistance of the shape of a sword is less than that of a human, not to mention the coeffcicent of friction of metal is lower than skin/clothing.

Along the same lines if the sword was to impact the boulder it could penetrate it for the stress at the tip of the sword would be greater than the molecular bounds that hold rock together, but the sword itself would be damaged unless it in itself is magical. Not to mention the time it would take to hit the ground from 6500 feet would be 20.2 seconds and be traveling (without friction) 646.4 feet per sec (~440mph).

a=32 feet per second squared

As per the sound it would be dependant on the distance and impact determined. If the sword fell through tree branches then in to the sand stone below then it would be less than a direct impact on the granite boulder.

Would be interesting to see how you would play the kinetic impact, but how much of that do you need to get past the sword in the ground bit to the actual story.

11-20-2004, 06:05 AM
the force of impact from a penny dropped off of the empire state building is enough to bury the penny in the concrete below. Pennies have horrible aerodynamic properties; much closer to those of a feather than of a sword.
(without friction)I think this answers the problem sufficiently. See, there is air all around the Empire State Building. Air is made of particles; particles that bang into the falling penny (or sword), creating friction and therefore terminal velocity. (See JR's comments above.)

Seems to me the only way we can have a significant event of the sword and rock, we must rely on magic. The requirements of physics (on Earth, anyway) gives us a somewhat mundane event.

Flawed Creation
11-21-2004, 12:52 AM
Terminal velocity is the point at which gravity and friction balance, correct?

if one threw something with some initial force, that wouldn't change the velocity it reached ultimately, correct? dropped or flung objects will top out at the same speed?

11-21-2004, 01:02 AM
That sounds right, Flawed. A shuttle reentry is a good example of an object that enters the atmosphere at a speed much greater than terminal velocity, but slows down drastically due to friction with the atmosphere.

11-21-2004, 02:14 AM
Thrown potato chips, too.

11-21-2004, 06:19 AM

MacAl Stone
11-21-2004, 08:04 AM
...I'm spending wayyyyyy too much time here, when i find myself experimentally flinging potato chips across the room to demonstrate HConn's assertion to myself....:\

11-21-2004, 12:37 PM
Terminal velocity is the point at which gravity and friction balance, correct?Yep. Gravity is a force. What we're talking about here is the result of that force on a body. Specifically, the force of gravitational attraction between the Earth (a very large body) and a sword (a relatively tiny body). The force acts upon both bodies, but the force of gravity from the sword is negligible, and in our discussion, can be considered nil. Remember that the force of gravity is normally expressed as an acceleration (9.8 meters per second per second). Since the force acts on the sword constantly, the sword would continue to accelerate were it not for friction.

Let's imagine a situation where there is no friction (in this case, that produced by air). For the sake of argument, let's say someone has vacuumed up all the atmosphere on Earth. NOW, if the sword is dropped, not thrown (initial velocity = zero), at the end of one second, the sword will have reached the velocity (v) of 9.8 meters per second. Two seconds later, v = about 20 m/s. A second after that, 40 m/s. Four seconds after being dropped, v = 80 m/s...and so on. There are two things, then, which would limit the sword's speed.

The first limit is if the sword strikes something (Earth, the rock, another angel...). The result depends on the distance between the place where the sword is released and the rock--the greater the distance, the more severe the collision.

The second limit is, of course, the speed of light. In the absense of friction, two bodies will accelerate toward one another until they reach the speed of light. And what happens then is ... I don't know what happens. Something bad, maybe. Definitely something worth writing a story about.

A Pathetic Writer
11-27-2004, 11:50 AM
I think I had this one on the SATs.

"If a sword falls in a forest, and no one is there to hear it, will it cleave a rock in two?"

11-28-2004, 06:56 AM
Only if someone comes and finds the cloven rock.

01-14-2005, 01:23 PM
Aramas is right about how the sword would tumble. Think of a sleek fighter jet - even if it starts to fall from a standstill, it's going to tumble and spin. The guy who just went into outer space designed his ship like a badmitton birdie - with lots of fins in the back to keep it from doing just that.

As for the impact, why not order a heavy-duty sword over the net and take it up in a Cessna? I'm sure you could get lots of people with safety goggles 8o and video cameras to tell you what sort of impact it made. If nothing else, the police would be happy to inform you at the trial.:D

More realistically, get a car going 70 mph and chuck a heavy strip of steel out the window at a ledge or tree.

A friend once knew a guy who made a heavy-duty crossbow. When it was finished, they loaded in a steel bolt and fired it. It whipped sideways, missed the target, and slammed sideways four inches deep into concrete blocks. :eek The guy immediately took the crossbow apart and never touched it again.

Personally, here's how I would do it - the angels, wishing to get this sword to the boy, drop the sword from the clouds. The sword spins and misses the stone. "Crap." They pick it up and try again. It gets hung up in a tree. "Damn." They try again. It kills a cow. Finally one of them, all pissed off, points at the rock and says, "Shazam." The rock splits, he leans the sword in the middle, shoves the two halves together, and leaps back into the sky.

XThe NavigatorX
01-16-2005, 12:21 PM
When I was a kid we used to make fake swords and armor from sheet metal, and after an extremely geeky session of D&D, we'd kick it up a notch and run around the woods in our getups waving our swords at each other.

My friend climbed a tree wearing his outfit with me standing below watching. His sword fell out of the sheath and headed straight for my head. I can still picture it tumbling out of the sky for me.