Darren New wrote:
> True. But to get the electrical propultion to work, the projectile needs 
> to be pretty small, which means it needs to go pretty fast. Otherwise 
> it's more of a thrower than a firearm. :-)

A railgun is exactly that - a thrower. A firearm uses chemical reactions 
to provide impulse.

You would never use one as a portable antipersonnel weapon anyway, it's 
probably more useful as a large scale space-based anti-ship or 
planet-busting weapon.

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Darren New wrote:
> (The railguns in the story actually had to fire 
> for a few thousand rounds to punch a hole in the atmosphere so the 
> needles wouldn't just vaporize.)

That's pretty cool. Relativistic weapons would only really be as useful 
as nukes - to cause almost-total destruction.

>> But anyway, even without relativity, the recoil would be absurd.
> Yep. If it can go thru a concrete wall coming out the front, the stock 
> can certainly break your shoulder.

It would do more than that. I can't be bothered with the sums right now 
but I imagine the recoil from a relativistic projectile gun would smash 
you to bits. You'd have to design any such gun to be recoilless somehow. 
<insert inertia-absorbing machinery here>

:)

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Bill Pragnell <bil### [at] hotmailcom> wrote:
> Darren New wrote:
> > Bill Pragnell wrote:
> >> although the basic premise for the gun is a real one, 
> > 
> > Sure. You try holding a weapon that fires something at relativistic speeds.

> I meant the railgun aspect - accelerating the projectile using 
> electromagnetics.

  Newton's third law still applies, even at relativistic accelerations.
If a force is applied to the projectile to make it accelerate at a certain
direction, another force will accelerate something else (ie. the gun)
at the opposite direction. I can't think of any way of avoiding that.

-- 
                                                          - Warp

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Warp wrote:
> Bill Pragnell <bil### [at] hotmailcom> wrote:
>> Darren New wrote:
>>> Bill Pragnell wrote:
>>>> although the basic premise for the gun is a real one, 
>>> Sure. You try holding a weapon that fires something at relativistic speeds.
> 
>> I meant the railgun aspect - accelerating the projectile using 
>> electromagnetics.
> 
>   Newton's third law still applies, even at relativistic accelerations.
> If a force is applied to the projectile to make it accelerate at a certain
> direction, another force will accelerate something else (ie. the gun)
> at the opposite direction. I can't think of any way of avoiding that.

Yes, I know all that. When I said 'basic premise' I was referring to the 
use of a magnetic field to accelerate the projectile, not the issues 
surrounding the claimed muzzle speed.

Hollywood gets projectile momentum transfer wrong all the time. Every 
time somebody gets shot by any kind of weapon, for a start.

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>  Newton's third law still applies, even at relativistic accelerations.
> If a force is applied to the projectile to make it accelerate at a certain
> direction, another force will accelerate something else (ie. the gun)
> at the opposite direction. I can't think of any way of avoiding that.

Bolt it to the Earth, then (assuming you're measuring recoil relative to the 
Earth) the recoil will be avoided.

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Bill Pragnell <bil### [at] hotmailcom> wrote:
> Hollywood gets projectile momentum transfer wrong all the time. Every 
> time somebody gets shot by any kind of weapon, for a start.

  Yes, it's funny how Hollywood physics are so skewed while still not
appearing completely wacky to the layman.

  For example, the faster the bullet, the farther it throws the person
who is hit by that bullet. In reality it's almost the opposite: The faster
the bullet, the less effect on the victim. The bullet just goes through
like a needle, and only little of its kinetic energy is trasnferred to
the victim itself. A slower bullet, however, transfers its kinetic energy
more to the victim because the body of the victim gets to slow down the
bullet, even so much that it might not even get through. In that case
the entire kinetic energy of the bullet is transferred. However, even in
that case the amount of kinetic energy is quite small compared to the
mass of an average person (the bullet is, after all, very light) and it
will not cause too much of an effect. Any minor effect will more likely
be overwhelmed by muscle spasms caused by the pain, which may cause the
victim to move to any random direction.

  (In some cases, if a fast bullet goes through, the jet effect caused
by the blood in the exit wound might even cause a movement towards the
shooter, not away from him... Still quite small, though.)

  Of course if we think about it in newtonian terms, if the victim gets
thrown back several meters, so should the shooter be too. The exact same
kinetic force is applied to the shooter, to the opposite direction.
(Well, technically speaking *more* kinetic energy is applied to the shooter
than to the victim because the bullet loses some kinetic energy during its
flight due to air friction, so if anything, the shooter should fly farther
back than the victim.)

  Then there are all those sparks. Bullets always send sparks when they
hit almost any surface. Of course real bullets don't, because bullets are
usually made of metals (such as copper) which do not spark even if they
hit some other metal. The movie makers themselves know this because they
can't get the sparks with real bullets but they must cheat (at least in
older and cheaper movies they used specially constructed "bullets" which
send sparks when they are broken).

  Those sparks have always bothered me. They make the scene unrealistic
and overboard. They don't add to the intensity of the scene.

  For a superb exception to this, see the beginning scene of Saving Private
Ryan. It's super-realistic. No bullet sparks. Yet the scene is extremely
immersive and intense. You can really *feel* the danger posed by those
bullets which you can't see but which are coming from everywhere.

-- 
                                                          - Warp

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Darren New wrote:
> Orchid XP v7 wrote:
>> range, and you can charge it up to deliver hits that can take down 
>> just about anything. (If you don't miss!)
> 
> You forgot "It shoots through walls", altho I must admit I never really 
> learned to take advantage of that feature, given the difficulty of 
> *aiming* thru walls.
> 

I remember that gun - excellent in DM games.

As others have said - one can get pretty good at where to shoot at the 
right times to get a kill.

I had some people pretty miffed at me sometimes because they couldn't 
hide anywhere...


Tom

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scott wrote:
>>  Newton's third law still applies, even at relativistic accelerations.
>> If a force is applied to the projectile to make it accelerate at a 
>> certain
>> direction, another force will accelerate something else (ie. the gun)
>> at the opposite direction. I can't think of any way of avoiding that.
> 
> Bolt it to the Earth, then (assuming you're measuring recoil relative to 
> the Earth) the recoil will be avoided.

That wouldn't half spoil the movie's storyline.

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Warp wrote:
> Bill Pragnell <bil### [at] hotmailcom> wrote:
>> Hollywood gets projectile momentum transfer wrong all the time. Every 
>> time somebody gets shot by any kind of weapon, for a start.
>   Yes, it's funny how Hollywood physics are so skewed while still not
> appearing completely wacky to the layman.

[snip detail]

Agreed - all of it. Maybe I read too much into realism, but I just can't 
take most action films seriously. The other things that are always wrong 
are explosions. A big pretty billowing fireball with hardly any force 
behind it is definitely not what a hand grenade does, or indeed any 
other explosive. A small firework and a gallon of fuel, maybe.

For a significant departure in this regard, watch The Kingdom. There's a 
car bomb explosion nearly halfway through that was done using real C4 
explosive, and it looks terrifyingly powerful. And very real.

That said, there are some very silly films that manage to mostly get 
this sort of thing right - True Lies, for example. Unlikely in the 
extreme, but most of the actual action isn't too far-fetched.

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scott wrote:
>>  Newton's third law still applies, even at relativistic accelerations.
>> If a force is applied to the projectile to make it accelerate at a 
>> certain
>> direction, another force will accelerate something else (ie. the gun)
>> at the opposite direction. I can't think of any way of avoiding that.
> 
> Bolt it to the Earth, then (assuming you're measuring recoil relative to 
> the Earth) the recoil will be avoided.

http://www.xkcd.com/162/

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