/sci/ - Science & Math

[Anonymous No.1934834]

Hi, not /sci/entist here, i have a question for you (sorry if it has allready been answered) :

if a guy shoots his AK47 vertically, could the bullet (let's say 8 grams of lead) hurt someone while going back down (sorry for my english) ?

I think you will answer in "joules" but if you can translate that in something understandable by everyone (like : it's like beeing hitting by a 1 kilogram ball droped from 10 meters or something...) it would be nice.

Thanks.

[Anonymous No.1934835]

please, please, please try this experiment out yourself. many, many times.

[Anonymous No.1934840]

Yes, you can be hurt, or killed. There are documented cases of this happening.

[not OP here !jepdZRdPkI No.1934850]

We had some TV-show here in Germany that had exactly that question. And they found real victims of exactly that occurrences. I can not recall the calculation but the could show that the bullet reaches ( I THINK it was) 2,000 meters and will then accelerate back with 9,81 m/s to the power of 2. They found several people that were hit by bullets that were shot high in the air and came back to earth on some totally different place. It is not lethal though.

[Anonymous No.1934856]

It would come back down at the same speed it left the barrel so it would kill you assuming no air resistance (in a vacuum). Realistically the air resistance would slow it down a lot. It would hurt but wouldnt kill you.

[Josef !!nUf2NflSAyw No.1934858]

There is a terminal velocity in each viscuous medium, such as water or air. This means that there is a certain speed moving particles' velocities converge to.
What happens when you're shooting straight up is that the bullet is travelling some distance, then stops for a moment, then starts falling again. This fall is an ordinary free fall, and the bullet would only reach terminal velocity in it, not necessarily the velocity it was fired with in the beginning. (I don't know how large terminal velocity is on Earth, but Google will help with that)

[Anonymous No.1934867]

no, it will come back down at the speed of gravity (9.8 meters per second), which is not lethal velocity. it would be just like dropping a piece of metal on someones head

[Josef !!nUf2NflSAyw No.1934872]

>>1934867 speed of gravity (9.8 meters per second)

[Anonymous No.1934874]

A relatively large bullet like an AK47 round would certainly hurt, if it fell on your head, but it would not penetrate the skull, so it would be unlikely to kill you.

[Anonymous No.1934878]

>>1934856

This.

[Anonymous No.1934884]

>>1934872
9.81 whatever, big fucking difference moron

[Anonymous No.1934888]

>>1934884
1) current theories suggest gravity, in the sense that it travels, travels at the speed of light
2) acceleration due to gravity is approximately 9.8 meters per second per second

dismissed

[Anonymous No.1934895]

>>1934884
Gravity is an acceleration, not a velocity. Jackass.

[Anonymous No.1934897]

>In the case of a bullet fired at a precisely vertical angle (something extremely difficult for a human being to duplicate), the bullet would tumble, lose its spin, and fall at a much slower speed due to terminal velocity and is therefore rendered less than lethal on impact. However, if a bullet is fired upward at a non-vertical angle (a far more probable possibility), it will maintain its spin and will reach a high enough speed to be lethal on impact. Because of this potentiality, firing a gun into the air is illegal in most states, and even in the states that it is legal, it is not recommended by the police. Also the MythBusters were able to identify two people who had been injured by falling bullets, one of them fatally injured. To date, this is the only myth to receive all three ratings at the same time.

From mythbusters

[Anonymous No.1934898]

My "common sense" tells me that it depends on how the bullet fell. If it fell point first, the velocity would be sufficient to kill you due to less air resistance. Also, the mass of the bullet would be concentrated into a small point. If the bullet was tumbling, the air resistance would decrease velocity substantially. Also, it may strike you on the flat side, distributing the mass over a larger area.

[Anonymous No.1934903]

OP here :

sorry i thought this was a scientist place and i could have a calculated answer, then translated in a common mortal language but it appears i was wrong...

[Anonymous No.1934918]

>>1934903
> OP here, I thought you guys would spoonfeed me an answer because I am dumb

[Anonymous No.1934949]

>>1934918

i'm not dumb, i'm scientifcally uncultivated

[Anonymous No.1934954]

>>1934949
read: im dumb with regards to science, so spoonfeed me the answer

[Anonymous No.1934955]

>>1934903
Calculate it yourself you lazy fuck.

[Anonymous No.1934956]

yep, if the bullet lands on somebody's head, it will have the same result as if the AK47 was put directly against their head before the trigger was pulled. on the way up, all the kinetic energy in the bullet is transfered into potential energy, at the peak height, the bullet isn't moving, and then begins to fall. at ground level again, all that kinetic energy has been converted back into kinetic energy, so it is travelling just as fast as when it first started (when it left the gun) so a person will die if hit. its just conservation of energy.

[Anonymous No.1934958]

>>1934949

[Anonymous No.1934967]

>>1934956
Later in your high school physics course you will learn about drag and terminal velocity.

[Anonymous No.1934969]

why do you need a calculated answer?

and well first you need to know the velocity at which the bullet exits the gun, the weight of the bullet - then you can figure out how high it's gonna get (leaving out friction and the like) and then use gravity acceleration to come up with an answer, pretty much.

[Anonymous No.1934971]

>>1934956
NEEDS FRESHMAN YEAR PHYSICS.

[Anonymous No.1934972]

>>1934867

>at the speed of gravity (9.8 meters per second)
>the speed of gravity
>speed

you best be trolling nigger. 9.81 ms-2 with the empjhasis on the -2, its accelleration due to gravity, not speed of gravity. also, this is me >>1934956
and i forgot about air resistance, as this guy said >>1934856
and the bullet would be slowed down, slightly, and lose some of the kinetic energy, but not much, the bullet will still have a considerable proportion of the speed it had at the start, aat the end, and will be lethal still.

[Anonymous No.1934977]

>>1934971
>>1934967
tsk tsk all bitching no credit given.

he's right in that if the lead hits you on the way down on the dead say, you're dead.

[Anonymous No.1934986]

to save you the trouble here's the wiki on your 8 gram bullet

123 gr (8.0 g) Full Metal Jacket velocity - 720 m/s (2,400 ft/s) energy - 2,045 J (1,508 ft·lbf)

[Anonymous No.1934991]

>>1934986
Muzzle velocity is irrelevant, it does not contribute to terminal velocity.

[Anonymous No.1934998]

>>1934967

yeh sorry i forgot about air resistance, i reposted somewhere up there correcting myself before reading you're post. and i know about terminal velocity, but the aerodynamic shape of the bullet means it has a very high terminal velocity, so it will still kill you and not be slowed down much by air resistance.

[Anonymous No.1935000]

>>1934991
Muzzle velocity would matter, because if it isn't high enough, the bullet will not reach terminal velocity.

[Anonymous No.1935013]

>>1934969
OPh:

because it seems more complicated : ok i can see that it's nothing but a piece of lead being dropped from the height you say but their is an air friction and maybe a density problem isn't it ?

>>1934958
lol, yes that's me and i want to know if i can fire my AK47 in the air for my daughter and brother's wedding.

[Anonymous No.1935015]

terminal velocity at 20 deg C ~ Sqrt[{2 8 9, 81}/{1204, 1 0, 295 3, 96 3, 96 3, 14}]

amirite?

[Anonymous No.1935017]

>>1935000
Technically correct, but irrelevant in practice. Every real gun has a high enough muzzle velocity that the bullets would reach terminal velocity on the way down.

[Anonymous No.1935022]

>>1935000

nice trips

anyhoo... either way it makes no difference, even if the bullet reaches terminal velocity on teh way down, at this point it is going fast enough to kill you anyway, and if the bullet wasnt going fast enough origionally, to go high enough to reach terminal velocity on the way down, it will have pretty much the speed at which it left the gun at the start, and presuming that an ak47 against the head, pull the trigger = killing someone, the person will still die. so they die in both cases.

..yeh.

[Anonymous No.1935037]

>>1935022
This is not true at all, terminal velocity of a bullet is much lower than muzzle velocity, often around 1/10th of muzzle velocity depending on many factors.

[Anonymous No.1935047]

theres a constant associated with different types of geometries in the terminal velocity formula which is determined experimentally for each type of projectile

[Anonymous No.1935052]

>>1935037

yes you're right, terminal velocity is far lower than starting velocity of the bullet, but both are still fast enough to kill someone.

[Rob No.1935057]

You want a concise answer?
Fine (even though mythbusters did this...)

Firing a 7.62x39 round from an AK47, SKS, or any of the many guns that fire this round, results in just the bullet leaving the gun at around 735 m/s (wikipedia)

As previously stated, there are two cases here, completely vertical, and at an angle. I'll do vertical first.

Vertical:
Fired straight up, the bullet will travel far into the air, decelerating at 9.81 m/s/s (yes, that is right, it is an acceleration, so velocity over time, learn calculus if you don't get it), but actually slowing down a bit more slowly as it travels further away from earth, but this is negligible, and can be safely ignored in this case, as air density, air pockets, wind, and other atmospheric effects have a much greater effect.

Now, the bullet will actually be slowing down much more quickly then this, due to air resistance. The bullet will go to a height of vy2 = uy2 + 2ayΔy (without calculating air resistance). When you put air resistance into the mix (which is a bit complicated, as it changes for the density of the bullet, its shape, its speed, and all the stuff I listed above) it actually only goes about 5 km up.
Continued below...

[Rob No.1935059]

At this point, the bullet stops, and becomes "unstable". It turns around and starts an unstable free fall, tumbling around and accelerating again at 9.81m/s/s. As it speeds up, air resistance again comes into play, and starts pushing against it. The bullet reaches what we call "terminal velocity", which is the speed at which the acceleration due to gravity matches the push of air resistance. This is different for all objects, and as mythbusters found, it is about 160 km/h or 44 m/s

The bullet continues to fall at this speed, until it hits the ground. Now, 44 m/s is around 100 miles an hour, so if you think of somebody throwing a small piece of metal, like a few quarters taped together, out a window of a car at top speed, you have about the amount of force this object has.

Continued below...

[Anonymous No.1935065]

>>1935022
A .30 caliber bullet weighing .021 pounds, comes down bottom first at a terminal velocity of 300 ft/s. That's a bit slower than the muzzle velocity of a BB gun. That's 40 joules = 30 foot pounds. It would definitely hurt like a mother, but it wouldn't kill you.

[Anonymous No.1935093]

Not lethal but injured.

Many many people in london were hurt from bullet shell casing from planes fight above, aswell as the AA fire coming back down, awell as the bullets from the planes which were lethal if shot downwards obviously.

Of course the bomb threat from the germans outweighed the damage of the bullets of friendlies

[Anonymous No.1935103]

>>1935065

>bottom first

like this? surely not,

i had a nerf gun as a kid (shoots foam missiles shaped like a bullet) and shot up high into the air they always turn around and aim nose down to the ground again.

[Anonymous No.1935131]

>>1935103
Yep, the same reason space capsules orient themselves that way. Your nerf missiles either had fletchings or a weighted tip.

[Anonymous No.1935138]

>>1935103
Real bullets move fast enough and are heavy enough that they tumble as they fall, like a coin. A tumbling bullet would fall slower than one falling bottom-first, so the 300 ft/sec is an upper bound for this type of bullet.

[Rob No.1935141]

You said you wanted numbers, so that is:
((44 grams) * .5) * ((44 (m / s))^2) = 42.59200 joules
or, in the weight of a baseball,
sqrt((42.59200 joules) / ((142 grams) * .5)) = 88.1733504 km/h
so it is equivilent to a baseball thrown by a half decent pitcher.

Next, angle... continued...

[Anonymous No.1935142]

>>1935131
Prove that bullets will come down base first.

Everything in my brain is telling me they wont. The physics generator in my brain.

[Anonymous No.1935144]

>>1935138
makes sense.

[Anonymous No.1935148]

>>1935142
That's what experimental results showed for that type of bullet. Probably longer bullets would be more likely to tumble.

[Anonymous No.1935160]

I assume that if that's a baseball being thrown by a pitcher it could kill?

assuming that during the tumbling of falling down the bullet dives in nose first.

[Anonymous No.1935165]

>>1935131

don't normal bullets also have a weighted tip? the middle bit at the base end is hollow, it is where the gunpowder was stored i think...so the tip weighs more.

[Anonymous No.1935179]

My god this thread is full of retards.

If fired perfectly vertically, the bullet should tumble and come down at terminal velocity. It would hurt if it hit you, but it is unlikely to be lethal. And it should fall in an area about the size of your backyard.

If just fired towards the sky, there is a real risk that it will follow an arced projectory, maintain its spin, and hit somewhere in a few mile range at deadly velocity. So you could kill someone in your neighborhood.

In general, it isn't really a good idea to fire weapons straight in the air. Even a nonlethal hit will still seriously hurt.

[Rob No.1935187]

Angle:

If fired at an angle, the bullet doesn't slow to a stop, and doesn't tumble through the air. Because the bullet keeps its spin, it actually has a much higher terminal velocity. It also gets to keep it's non-vertical component of its velocity, as only air resistance is pushing against it. The bullet will drop to subsonic, and it will certainly not penetrate like a direct hit, but it will still have a good kick. It really all depends on the angle.

The best way to draw a similarity here, is to look at the distance calculations on any gun with sights. You adjust the sight on the target to make it so that when you aim, you are pointing the barrel above what you are shooting at. This is because the bullet will start to be pulled down by gravity. The bullet travels at an upward angle first, but because it is being pulled down by gravity, loses the upward speed, and starts traveling downward. This happens really fast, and because the bullet is traveling SO fast horizontally, it hard to imagine. This is the same effect you see when you fire at an extreme angle.

Without doing calculations for air resistance, I don't have any numbers on this.

Done. Hope that finished your fucking homework for you.

[Rob No.1935196]

Somebody check my math on the baseball thing... it seems too high.

[Anonymous No.1935198]

>>1935187

that is no homework but i thank you anyway

captcha : those prondays

[Rob No.1935215]

Shit, I did do the math wrong. I had the wrong mass for the bullet, by a LOT.
sqrt((((8 grams) * .5) * ((44 (m / s))^2)) / ((142 grams) * .5)) = 37.5972429 km/h
baseball going 37 km/h.
My bad, sorry

[Anonymous No.1935228]

>>1935179

>my god this thread is full of retards.

fuck you

>projectory

what? i think you mean 'trajectory' or maybe you were crossing it with the word 'projectile...
who's the retard now?

what out for trajectiles on you're way out.

[Rob No.1935245]

>>1935228
No, fuck you. Fuck you for saying "fuck you" to the guy trying to dispel misinformation.
Fuck you for stating information like it was fact when you are completely wrong.
Fuck you for picking apart a typo or misspelling when that really isn't the point of a sci board.
Fuck you for being a bitch.
Fuck you for being a disservice to what science is.
Fuck you with all the possible meaning of "Fuck you"
Now kindly Fuck off.

[Anonymous No.1935254]

>>1935245

[Anonymous No.1935256]

Am i doin it right? huurrhhh duhhh??

[Anonymous No.1935258]

>>1935254
arguing is hard, eh?

[Anonymous No.1935287]

>>1934858
>the bullet would only reach terminal velocity

>mfw terminal velocity is the speed that a falling object approaches but never reaches in free fall

learn some differential equations, dumbass

[Anonymous No.1935290]

>>1935258

not at all, you just happen to suck at it. i'm on around tier 3, while you're clearly some rude unintelligent guy who is on about tier 7. pic related.

[Anonymous No.1935305]

>>1935258
>>1935290
will you two failtards stop bickering for a moment
and tell me if I did this right? >>1935256

[Anonymous No.1935309]

>>1935305
stfu, I made one freaking post
retard

[Anonymous No.1935330]

>>1935309

Oh sorry.. witch one was yours? ohh and did I do math
correctlY?

captcha : example quermeno

[Rob No.1935342]

>>1935305
I haven't done anything but look at the final result... That is way too slow. Think about the number. That's a slow jog, a bullet falling is gonna be a lot faster then that.

[Anonymous No.1935343]

>>1935179

so you think firing vertically up will not kill someone, but firing at an angle is more likely to?
this is wrong, we learned about moving projectiles in mechanics, their is a verticle component an a horizontal component of the velocity, if fired vertically upwards, the bullet goes higher, so it falls further and has more time to build up speed.
fired at an angle, the verticle component of velocity is lower, although it does now have a horizontal component. a2 + b2 = c2, so you can work out its overall velocity if it is fired at an angle, but its no more likely to kill someone than the bullet fired completely verticle

[Anonymous No.1935354]

>>1935342

yeh that number is far too low i'll check through the working in a sec... it's been a while since i did this kinda stuff tho.

[Anonymous No.1935355]

This is like basic physics, it will have the same velocity back down as it did up.

So yes, just as fucking dangerous as shooting someone point blank

[Anonymous No.1935369]

>>1935355

yeh thats what i said, here >>1934956

but we both forgot about air resistance, so teh bllet will be slightly slower.

[Anonymous No.1935371]

>>1935287
>assuming terminal velocity is constant over time

If the bullet tumbles, its terminal velocity is constantly changing, and it can indeed meet or exceed terminal velocity.

[Anonymous No.1935375]

>>1935342

I was thinking the same thing. you would expect it
to be falling at 30km/hour at least. But as noted
I think it has to do with the surface area not being
exact enough.

[Anonymous No.1935387]

>This is like basic physics, it will have the same velocity back down as it did up.

[Rob No.1935389]

>>1935355
>>1935343
JESUS! Stop posting if you don't know what you are talking about! If you haven't had at least 1st year university education in physics, SHUT THE FUCK UP! I know you THINK you know what you are talking about, but you don't. You are just shitting on this thread and breaking the minds of people who MIGHT learn something. Read my above posts. There are a few others in this thread who understand, and explain as well.

[Rob No.1935400]

>>1935369
A LOT SLOWER

[Rob No.1935407]

>>1935354
Oh yeah, given your understanding of basic physics, I can't wait to see you do math.

[Anonymous No.1935408]

>>1935354
8gram ... you have to convert it into kg.

Also, if you are converting from m/s into kmph you don't times it by 60. You times it by 60 to get it into minutes and then 60 again to get it into hours. So times it by 3600.

(my captcha had 3600 lol)

[Anonymous No.1935412]

>>1935354


http://en.wikipedia.org/wiki/Terminal_velocity
http://en.wikipedia.org/wiki/Density#Density_of_air_.28at_1_atm.29
http://en.wikipedia.org/wiki/Drag_coefficient

sauce of the formula density and coefficient. I chose
the angled cube due to its likeness to a tumbling
bullet.

[Anonymous No.1935421]

kk i can't quite tell what you've done... erm. yeh for the mass, you need to convert to kilograms i think, so 8 grams is 0.008 kilograms, so have m as 0.008

[Anonymous No.1935449]

>so you think firing vertically up will not kill someone, but firing at an angle is more likely to?

If you fire something straight up it is going to spend a longer time moving through the fluid (air), so it is more likely to reach terminal velocity/it will be more affected by drag.

[Anonymous No.1935451]

>>1935408

yahh that ends up being 0.1 km/hour.........

[Anonymous No.1935455]

>>1935408

damn, you beat me to it.

and fuck you rob, you think you are so clever but you are just an arrogant dickhead, my science is sound.
air is very thin, for something very dense, small, and aerodynamic like a bullet is has a fairly negligable effect. objects that are light hand have large surface area are far more affected by air resistance.

you point out where you think i'm wrong and lets go through it, shall we?
apart from that, shut the fuck up!

[Anonymous No.1935479]

>>1935256
lol you tard. How the hell did you get 188m^2 of projected surface area? Should be approximately a=pi*(0.0005^2).
(0.5 cm radius) guessing from OP's pic. And m is 0.008 kg.

[Anonymous No.1935482]

>>1935449
Velocity at impact with ground is a function with one value with respect to angle fired. At 0 degrees its essentially muzzle velocity, and at 90 degrees its terminal velocity. We also know that its monotonically decreasing - higher angle means less speed due to more drag.

At some angle the bullet is going to hit the ground at terminal velocity without ever going slower than terminal velocity. Angles higher than that are basically equivalent, while angles lower than that contribute to more speed on the bullet.

[Anonymous No.1935486]

>>1935451
Check your other variables.
I think you forgot to times your radius by pi for the area.

[Anonymous No.1935487]

>>1935389
Rob, I think the idiots continue marching on because they believe an exact answer is possible and won't be satisfied until it is found, as they have been trained to do by their schoolwork. They don't understand that a tumbling bullet is a chaotic system and an approximation is the best we can do.

[Anonymous No.1935490]

>>1935256
lol you tard. How the hell did you get 188m^2 of projected surface area? Should be approximately a=pi*(0.005^2).
(0.5 cm radius) guessing from OP's pic. And m is 0.008 kg.

[Anonymous No.1935497]

>>1935389
Lmao.Ok jackass. Because the relatively small force of air resistance is going to be the deciding factor in whether someone could get killed or not.

OP asked whether or not people could be killed by an ak-47 bullet that was fired straight up and came back down, and they can.

Suck my dick.

[Anonymous No.1935500]

And its starting to look allot better. we are up to over
100km/h almost dangerous

[Anonymous No.1935524]

>>1935487

> the idiots continue marching

i hope you are not refering to me, i'm not trying to get an exact answer, obviously we are after just an approximate answer, as factors like how windy it is that day will affect the bullets final speed, as well as the altitude youstand when the bullet is fired.

[Anonymous No.1935530]

>>1935500
Nope. You mulitplied by 3600 but you still need to divide it by 1000 to go from metres per hour to kilometres per hour.
Also, use the value for area that the above poster talks about. A smaller area will give you a higher terminal velocity.

[Anonymous No.1935538]

>>1935524
A good approximate answer has already been given, why are you still at it?
>>1935057

[Anonymous No.1935539]

>>1935490

188mm of surface is approximately 2cm^2

look at a bullet, think "could I wrap a stamp around
this.......... yes I can..... my got that faggot might
be on to something" come back and give me an
apology.

If I miss marked something as m^2 instead if mm^2 I
am deeply sorry and you have my apology.

[Rob No.1935540]

>>1935455
I tire, but ok..
You said it would be almost the same as point blank in >>1935022
My calculations, and the reasoning of the other smart people here, have proven YOU wrong. You have not presented any analytical thoughts, just conjecture. I can't argue your "thoughts" or "gut instinct". I have provided my logic, and my math, you are free to debate any step I've taken therein.

My knowledge also comes from the fact that I have an SKS not 5 meters away, which fires this exact round, and I have an idea of how the ballistics work.

[Anonymous No.1935549]

>>1935539

lol hue is failing on my behalf? i can do that just fine
myself thank you...

[Anonymous No.1935563]

and FFS, it WILL kill you, i can drop a bullet from a tenth floor window, and if it lands on someones head on the street below, they will die, and a bullet fired into the air falls much further than this.

[Anonymous No.1935569]

>>1935539
you need to be in metres, not millimetres. Units are important.
Those equations only work when you use SI units. Otherwise your final answer won't be in metres per second.
The SI units for distance are metres.

[Anonymous No.1935586]

>>1935490
>>1935490

Thx 4 the information. Integrated it and it looks good.
the diameter of the thing should be 7.62 as that is what the AK fires. So I'm keeping that unless there are protests?

[Rob No.1935594]

>>1935497

Yes, the small force of air resistance, over 30-60 seconds, will have a huge impact on the ballistics. Cool eh!?

No, it will probably not kill you. I mean, we are fragile beings, if it hit the back of the neck, and you have a some condition that makes you vulnerable there, then yeah, maybe... but most of the time, if fired straight up, nope, you are safe.

[Anonymous No.1935608]

Your final answer will be a velocity. And you will have to state your assumptions (that the bullet reaches terminal velocity, what assumptions you made to calculate the area etc.)
If you want it in joules. Use E = 0.5*m*v^2 (v is the terminal velocity you are trying to work out, it has to be in m/s)

[Anonymous No.1935610]

HERE WE GO! looking really good now thx to the
fellow with the pi and the surface area info :D

[Anonymous No.1935618]

>>1935586
If that's 7.62 mm you need to divide it by 1000 to get it into metres.
7.62 mm = 0.00762 m.

Then you can put that number into your equation.

[Anonymous No.1935620]

>>1935608

one step at a time dude... we need to stop screwing up
the velocity first :D

[Rob No.1935622]

>>1935563

No it will not.
You do realize that the bullet is just the small copper bit up front on the OP pic right? The rest has just the charge and primer. Even if you dropped the whole thing, it wouldn't kill me from 10 stories! I'd definitely feel it, and yell "what the fuck, who's dropping shit!"
Drop a full clip, might do real damage from 10 stories, a case of 1100 rounds would kill me for SURE. But one bullet? Not even a full round? No fucking chance.
I'm not even going to do the math on that. This is just absurd. Common sense revolts the idea.

[Anonymous No.1935628]

>>1935539
WTF?
Have you ever heard of natural units? The natural unit for surface is m^2 not (mm)^2. You are putting in 188m^2 in your formula. Also it is meant to be the projected surface area. That's the area you see when you look up to the falling object from below. In this case it's a circle with the radius of the bullet. An to get from m/s to km/h you have to multiply with 3.6

[Anonymous No.1935647]

>>1935569

u shure bout this because when i change from mm to m
i end up with a Vt of march 7 :S

[Anonymous No.1935657]

>>1935610
You are still calculating the velocity of a bullet which has a radius of over 7m. Are you firing a nuke?

>An to get from m/s to km/h you have to multiply with 3.6

Yeah, he's got to that point already. Multiplied by 3600 and divided by 1000.

[Anonymous No.1935678]

Not from /sci/ but curious too... if the bullet travels upwards and loses momentum won't it have the exact same force once it reaches the height of the barrel?

If I throw a ball in the air at 90mph when it reaches back my hand it'll come back at that speed, no?

[Anonymous No.1935685]

>>1935678
Only in vacuum. But on earth we have air resistance. So no.

[Anonymous No.1935697]

>>1935657

yahhh---- nukes would be nice.. lets do that once
we manage to do this right....

[Anonymous No.1935704]

>>1935622

v = root {2XdistanceX9.8] (distance in meters) and before you go bitching about air resistance again, change the fucking record, it is negligable.

i said 10 stories, im presuming about 4 meters per story, give or take, so root[2X9.8X4X10] = 28 ms-1 approximate velocity (admitedly slightly less, as this doesnt happen in a vaccum IRL)

28 meters a second is fucking fast! and the bullet has a small surface area so it excerts a fuckton of pressure when it connects with something.

[Rob No.1935709]

>>1935697
HAHAHA

I love that about physics though, you can usually figure out if an answer is correct based on the absurdity of the number you get.

[Anonymous No.1935710]

When I consider this, I see two scenarios:

1) The bullet is fired directly up. It has been shown that the bullet will simply tumble on the way back down at non-lethal velocity. (Mythbusters).

2) The bullet is shot with some horizontal component to it's trajectory, or the wind imposes a similar trajectory in the "straight up" scenario.

This is where the bullet would be most likely to do damage. The forces acting on the bullet are gravity and drag. The velocity of the bullet will be less than the muzzle velocity (a well known conclusion). The major factor will be the drag of the bullet. As you are no doubt aware, this can be calculated from the geometry of the bullet, the air density, and the velocity. If a scenario allows for the force of drag to be small enough that the bullet maintains a lethal velocity, then of course it may be considered lethal.

Essentially, the only scenario I can think of where velocity of the bullet would be anywhere near lethal is a case in which the bullet maintains the spin imparted to it from the rifling in the barrel (i.e. doesn't tumble). This is probably true for 99% of real scenarios, which is why shooting guns into the air is either forbidden or heavily frowned-upon.

This is simply my interpretation of the problem.

[Josef !!nUf2NflSAyw No.1935715]

>>1935704
<div class="math">\ddot x + \beta\dot x^2 + f = 0</div>
... crossing out the beta in a nonlinear DE? Learned that from Dr. Werner, huh?

[Anonymous No.1935721]

oh and rob, the mass has no effect on the velocity in case you were wondering, regardless of the weight of the bullet, it will be travelling at 28 meters per second, i can drop the bullet and a bowling ball off the 10 storeys at the same time, and they will reach the ground at the same time, and have the same velocity.
the mass just affects the momentum they exert when they land, the bullet is made of dense metal, its heavy enough, and small enough SA to exert a lot of pressure, as i already said.

[Anonymous No.1935730]

Perfectly straight up and down, no. The terminal velocity for a tiny hunk of blunt metal isn't going to be so high it can do lethal damage unless you're in the middle of brain surgery or something.

However if it's a little off vertical the bullet could go substantially faster than terminal velocity when it returns, and kill somebody with an unlucky hit location. A bit of trig shows that we get up to ~100 m/s return velocity if fired at 80 instead of 90 degrees

[Anonymous No.1935731]

>>1935709

i think i got it... its the divide by 1000 i forgot from earlyer

[Anonymous No.1935741]

>>1935731
What's with th ".6" in the denominator?

[Anonymous No.1935756]

>>1935721

might I interject.... you forgetting one simple point.
Being shot by a bullet at long range is very much
survivable. If you are far enough away from a shooter
you may very well survive the impact of a bullet meant
to kill you because it doesn't have the mass to
contain the required kinetic energy to penetrate deep
enough. Simple fact of life is that bullets stop due to
air resistance in roughly the same amount of time it
takes for the cartridge to hit the ground.

[Rob No.1935758]

>>1935704
From 10 stories, air does not play into it that much. I'd probably say 27 m/s if I were to guess, but fuck it, close enough.

28 m/s is quick. Its almost as fast as I go when driving, skiing, or other not really that fast things.

I could catch the bullet at that speed. In my hand. I could catch it. It would not go through or anything.

Because this is such an easy speed to imagine, as it is about the speed of most highways, we can draw examples. If you were to throw the bullet out the window of a car, and hit me, it will sting, but not kill!

A paintball goes at 330 km/h out of the gun, and has not THAT different of a size from a bullet, but the chance of a paintball killing you are low, unless they hit your eye.

If I throw a snowball while going 110 km/h, it will not cause a crater, it simply isn't that fast.

[Anonymous No.1935761]

>>1935741

good question... where the fuck did that come from
i didn't put it there

[Anonymous No.1935776]

there we go

we should be able to start calculating some Jules now
unless we have any objections.

[Anonymous No.1935783]

>>1935758

oh yeah...
kk, thats about 100 kilometers per hour isnt it? i think you might have trouble catching it... hitting your hand it would still hurt like fuck, but yeh, you are right, it wont kill you, not from 10 storeys high.

[Rob No.1935785]

>>1935721
This is an often stated, and oversimplified view.
Please read http://en.wikipedia.org/wiki/Terminal_velocity
Mass does come into the equation, as does the size of the object. A plastic bowling ball will fall slower then a entirely metal one.

[Anonymous No.1935789]

how the fuck does this thread have 115 posts with no real conclusion lol

[Anonymous No.1935796]

OPh

>>1935731

thanks for trying, if your calculus is right and the cynetic energy is : EC = 1/2 X (m X v²)

we would have 0.5X0.008X266.8² = 284.7 joules which is higher than the muzzlz energy of a 9mm parabellum

pretty deadly...

[Anonymous No.1935799]

>>1935776
I'd use a drag coefficient of about 0.4. It's probably closer to a bullet than that 0.8 of an angled cube

[Anonymous No.1935805]

>>1935789
You're not paying attention, I guess.

The conclusion is "no", the rest of the posts have been nit-picking/hammering out details.

[Anonymous No.1935806]

>>1935057
>only 5 km up

it seems to high for vertical shot

the G3 rifles we shot in the army had a maximum range of 4 kilometers in a straight line

[Anonymous No.1935814]

>>1935796
sorry, higher than a 9mm short...

[Anonymous No.1935822]

>>1935758

you know backing your arguments up with some simple
facts might help your arguments.... because when
we are dealing with things like bullets we actually have allot of super good data right at our finger tips.

You don't need to guess at the speed of a bullet meant
to kill, because we know it, its 838 m/s. because that
is the muzzle velocity of a 7.62 NATO round. And
with this data you could be a smart ass also ;) and
use the logical augment that that speed... the speed
to kill is not twice as fast as speed from 10 storys..
nor is it three times as fast... no no... its 31.03 times
as fast... that's how much more speed you would need
to make it kill you effectively...

[Anonymous No.1935830]

>>1935799
Yep Wikipedia eve gives a value of about 0.3:
http://en.wikipedia.org/wiki/Drag_coefficient#Drag_coefficient_cd_examples

[Anonymous No.1935836]

>>1935822
But you can kill people by hitting them with a .30 cal bullet going much much slower than the muzzle velocity of .308

[Anonymous No.1935840]

>>1935799
Yep, Wikipedia gives a value of about 0.3:
http://en.wikipedia.org/wiki/Drag_coefficient#Drag_coefficient_cd_examples

[Anonymous No.1935847]

>>1935796

where did you get your data? my data says that your
mussel energy data is off by half...

[Anonymous No.1935852]

>>1935822

so you are presuming that the 838 ms-1 is exactly the bullet speed required to kill a person.
you're correct that it is 31 times more than 27. (ish) but so what?
it depends where the bullet hits the person, in the head, or heart, is going to be lethal, in the foot or arm is not.
also, you referenced robs post, but i think you are supposed to be arguing against me... i'm not sure.

[Anonymous No.1935860]

>>1935840
>>1935799

it was assumed that the bullet was tumbling in the air
and didn't provide a streamlined surface. do we have
some objections to this?

[Anonymous No.1935863]

>>1935847
see >>1935814

[Anonymous No.1935874]

>>1935785

lol i already have that wiki page open ;)

and i suppose this is down to your best friend; air resistance.

ive seen the video of the feather and the bowling ball being dropped on the moon, they hit the ground at the same time, it was pretty cool. but yeh, no air resistance on the moon.

[Anonymous No.1935875]

>>1935863
I see.

[Anonymous No.1935879]

>>1935875

as do i.

[Anonymous No.1935884]

I tried to work out the pressure the bullet would exert on you when it hits, assuming it falls at the velocity someone above calculated (57.4 m/s, seems about right).
I got about 100kPA, which is about the same as 1 atmosphere of pressure. This must be wrong.

To work it out I did
pressure = F/A
I used the area that the above poster used, (a radius of 0.00381m).
For the force I did
F=ma
m is the mass (0.008kg).
For acceleration I did:
dv/dt. The change in velocity is 57.4 and for the time I used 0.1 seconds.

Putting all this in gives the 100kPA. Can anyone point out what I did wrong?

[Rob No.1935887]

>>1935806
Yes, but the bullets you shot were not actively fighting gravity. They would start being affected by gravity as soon as they left the barrel. The bullets would travel the 4 km horizontally before they would hit the ground, but not travel directly opposite the direction of the force of gravity.

I would actually be surprised at a bullet going 4 km, unless fired at an upwards angle to maximize the distance.

[Anonymous No.1935890]

>>1935860
OK.

>>1935796
You're using km/h instead of m/s in your 0.5*m*v^2.

[Anonymous No.1935893]

>>1935852

actually hitting someone from that height, even when
using a gun and trying to kill the sucker is like a
1 in a 20 shot, for a laymen. then trying to hit someone with a bit of metal from that height, and
trying to miraculessly hit a magical critical weak
spot is also pretty dam hard, what we should do is
be fair and give the target a plastic hard hat. Then we
can say. If it kills the target wearing a hard hat, it will
kill fucking everyone...not just your "special test
subjects" with unusually soft skulls, or special
"weak spots".

[Anonymous No.1935903]

OPh :

sorry but beside the air friction, the air being a fluid isn't there some Archimedes push somewhere in the equation ?

[Anonymous No.1935945]

>>1935890
fuck, thanks :

0.5X0.008X74.1² = 21.9 joules

the power of an air rifle...

[Anonymous No.1935948]

>>1935893

wait, wtf is this? that '1 in 20' shot is just a number you pulled out of thin air, with no evidence to back it up, secondly, i was never even talking about whether the event was likely to happen, we were just working out that if it DID land on someones head, would they die.
third, dont just start inventing new variables like our 'target' having a hard hat, it doesn't help thing.

[Rob No.1935951]

>>1935903
Archimedes Principle states that the buoyant force on a submerged object is equal to the weight of the fluid that is displaced by the object.

That? that only comes into effect when the densities are near eachother. I mean, I could put it into the math, but negligible is key here.
You can throw any number of extra forces at it.. Pull from the moon and sun, relative time due to velocity being closer to C of one observer or another, etc etc.. but really Gravity and air resistance are what come into play here.

I think this is what causes a lot of confusion in physics. People don't know when to take variables out of the equation. Throughout science classes, we ALWAYS left air resistance out, because it makes it a lot easier to work with, and learn. You can always add more, but I think we've come to a reasonably exact answer, considering where we are.

[Anonymous No.1935969]

Ok the results are in

[Anonymous No.1935971]

>>1935951
ok

[Rob No.1935977]

>>1935948
I agree. The discussion was about if it hit you. If you thew 100 bullets off the 10 story building into a crowd, you'd probably hit a few people.

>>1935893
And you could totally hit a person at 40 meters with iron sights. With a scope you could score a head shot. You are NOT on my team come zombie apocalypse if you shoot that badly.

[Anonymous No.1936004]

>>1935969

>hitting something with a hammer
>253.5 joules

wut? you cant put an exact uon a hammerswing, thats completely silly, anyone could hit with many different levels of power each time, and some people are stronger than others

[Anonymous No.1936011]

>>1935969

170 joules is near the muzzle energy of the .32 ACP (180 joules), deadly.

[Anonymous No.1936015]

>>1935977

he said 'for a layman'

..perhaps he is not. military background perhaps?

[Anonymous No.1936046]

>>1935948

might I point out you where the one that bought up
variables to begin with, arguing that the muzzle
velocity is a minor factor compared to where it hits.
But the thing is if you have to hit just the right spot
to make it a kill when you drop it, but if you instead
fire it, you will kill on anything other then a toe,
finger or grazed shot. especially from that angle the
projectile from a gun like that is gonna tare him
apart. Where as dropping it as, said once already is
almost impossible if you do not hit him just right.
In which case you might as well strike him with a
spoon "just right", which has just as small a success,
unless your a specially trained spoon ninja.

[Rob No.1936056]

>>1935969
>>1934834
You calculated using km/h, not m/s
True answer using his numbers: 13.18 joules
If I redo my equation, (again really sorry for using 44 grams way back, got mixed up.)
((8 grams) * .5) * ((44 * (m / s))^2) = 7.74400 joules
So somewhere between those two, depending on the speed of the bullet at terminal velocity.

[Rob No.1936064]

>>1936046
lol, I like the idea of spoon ninja

[Anonymous No.1936069]

>>1936015

I said layman because, I know some people cannot
shoot for shit, like untrained young girls, allot
of children, and most Taliban fighters. (Taliban with
scopes on the other hand can be.... "a nuisance")

[Anonymous No.1936094]

>>1936064

dude don't fuck with the spoon ninja ... they will fuck
you up.... they fucking dangerous...

[Rob No.1936103]

>>1936069
Ah, you did say laymen.. I take it back.
If you are military, thank you.

[Anonymous No.1936107]

>>1936046

well i was kinda thinking it would just land right slap-bang in the centre on the top of their head.

and if the bullet is fired, it wont be a definite kill unless it hits vital organs, not just grazing shots are survivable. if you hit pretty much anywhere in their limps, shoulder, hips etc then the probably wont die, you would need a headshot, or a shot to the body damaging vital organs there.

[Anonymous No.1936109]

>>1936056

yahh i just re did it.. 13 J ... that's .... tiny.... can that
really be true i wonder...

[Rob No.1936139]

>>1936107
This starts getting into things like hollow point bullets. The stuff I mentioned is all relating to Full Metal Jackets, which most(all?) 7.62x39 are.

Hollow point ammo can seriously mess up a person, even if non-vitals are hit, considering how much they rip apart. I do not know much about this though, so I'm going to leave it at that.

[Anonymous No.1936149]

>>1936107

you absolutely right.... I would need to hit the head,
organs, large arterial things and junk like that....and
normally when firing from straight on that can be
kind of tricky.. but in this case you need to take a
birds eye perspective.... what can i see from up there
what can i hit going straight up and down.... i got his
head.... and i got his shoulders...and a bit of his
elbow....

in other words i almost cannot miss something vital,
unless i graze his elbow. You see evrything below
his shoulders is organs, and evrthing below his arms
is armpits... and armpits have two of the 6 biggest
atrial junctions in the body, shoot him their the bullet
tumbling down his innards opening up blood vessels left and right.. suckers gonna die... and painfully at
that :(

Kind of makes you wish you had chosen the spoon.

[Rob No.1936151]

>>1936109
Yup, down from over 2000.
AIR! It really does a lot.

[Anonymous No.1936162]

>>1936151

lol

"stick your face out window when driving along..."
"prove air is not imaginary force"

[Anonymous No.1936175]

OPh

it's 3AM here, going to bed

hope this thread will still be here tomorrow

[Anonymous No.1936178]

>>1936175

lol your right.. I gonna bail also... :D nite all...
bware of the spoon ninja

[Anonymous No.1936181]

>>1936149
There is no spoon.

[Anonymous No.1936185]

>>1936149

there is no spoon

[Anonymous No.1936193]

WTF!?

..FFFFFFFUUUUU!!!

[Rob No.1936216]

>>1936185
It was me who posted the neo pic...

I can't help but think about how far we've come from "FUCK YOU'

[Anonymous No.1936228]

>>1936216

yeh i no, rite?

i mean at first you seemed like an arrogant asshole, but your actually really good at science and stuff arn't you.
I actually learned something today. anyway i think i'll go to sleep now. goodnight.

[Anonymous No.1938027]

good morning *yawn* OMG this thread is still here? wow i guess threads of /sci/ last far longer than threads on /b/

erm...bump?

[Anonymous No.1938448]

>>1934834
myth busters proved it cannot happen
terminal velocity = it will pierce skin but not crush bone good day to you sir