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Passive Rail Gun

rare earth magnets are fun!
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Take an extraordinarily long track ,either a pipe or some sort of walled track a ball bearing could roll easily through will little drag, several hundred ball bearings, 1/2 or 3/4 inch, and a few hundred rare earth magnets. the idea is this, align the magnets at calculated (i dont know the equations, but I'm sure they're out there) intervals so that when the ball bearings leave one side of the magnet, they are immedately pulled to the next magnet.

lets try an illistration.

O=ball bearing II= magnet.

O_____IIOOOO______IIOOOO __________IIOOOO

O>___IIOOOO______IIOOOO __________IIOOOO

_____OIIOOO__O>__IIOOOO __________IIOOOO

_____OIIOOO______OIIOOO ___O>____IIOOOO

_____OIIOOO______OIIOOO _________OIIOOOO->

so the fisrt ball is rolled to the magnet, accellerated into it, so the energy of that colission is transfered through the first three bearings of the other side of the magnet and launches the 4th magnet, which is the accelerated toward the next magnet ect.

not sure if there is an upper bound to the acceleration from the magnets or if this would work, so anno away all comments are welcome.

bleh, Sep 14 2006

The video that I imagine bleh has seen. http://www.metacafe...ch/20901/metal_gun/
[st3f, Sep 14 2006]

(?) Baked. http://demo.physics...ipt.idc?DemoID=1098
They call it a Gauss rifle. [spidermother, Sep 14 2006]

[link]






       I'm not sure where the limit is, but I've seen it demonstrated and the science is sound. Is there a new idea here though?   

       (edit: that is, find a novel use for it, maybe drilling holes in tungsten plate or something, to justify the idea's existence. I for one like it)
david_scothern, Sep 14 2006
  

       Umm, as I understand it, the magnetic force operates on an inverse cube law, so it's effect never "ends," it just gets weaker and weaker. I'd imagine that if the ball bearings are accelerated at all, they'll be accelerated only to a particular magnet, or just maybe between two magnets.   

       I believe that most rail guns require electromagnets to be turned on and off at alternating times, or to reverse polarity, and the fluctuating magnetic field is what accelerates the ammo, so a passive rail gun, as you describe it seems to border on perpetual motion territory.   

       Of course, my science isn't exactly up to rocket-engineer level, so I'll refrain from boning until my hunch is verified.
ye_river_xiv, Sep 14 2006
  

       [YRZ] has it right. Otherwise you have created a perpetual motion machine.
Galbinus_Caeli, Sep 14 2006
  

       This is feasible, and will work. It won't be nearly as effective as the active rail gun for the reasons brought up by [ye_river-xiv].   

       The problem you face is that since you can't turn the magnets off, you can't stop them interfering. So the magnet forces won't sum directly. It's certainly won't be perpetual motion, it's a poor rail gun.
Jinbish, Sep 14 2006
  

       Not true G_C; google for it and you'll find it.   

       What happens is that the first ball bearing is rolled into the magnet. Magnetic attraction pulls it in (it loses magnetic potential energy) and it transfers its kinetic energy through the magnet and the string of ball bearings, to the fourth ball in the diagram. This ball, being the last in the string, has nowhere to transfer the KE to and is also relatively far from the magnet, so it can escape the magnetic field and does so with a lot of KE. It heads for the next magnet, which accelerates it on the way in, and transfers all the KE it has (including that provided by the second magnet) through the next string of balls. Repeat until the last ball is going very fast.   

       That last ball has all of the magnetic potential energy lost by the balls which are now stuck to magnets, plus the little KE you gave it by rolling at the start. No perpetual motion.   

       [Jinbish], they'll more or less sum directly, because the KE is being passed to a ball that is relatively far from the magnet. The magnetic forces will cancel one another out, except for the period when the incoming ball is closer to the magnet than the outgoing ball (three diameters or so in the four-ball case). As the magnet follows a 1/x rule (I think? it's inverse, in any case) the force exerted while the incoming ball is within those three diameters is significant.
david_scothern, Sep 14 2006
  

       I see - like the office toy. In this case you rely on the transferral of KE being close to an elastic collision and the influence of the magnets being inverse square. So, assuming no friction (!) and elasticity, you should gain energy proportionally to :
f(no. magnets, diameter*no bearings) or something similar...
Jinbish, Sep 14 2006
  

       Ok, I guess I was not completly understanding what was going on. Calling it a "rail gun" is really confusing me. It looks vaguely like a sort of interrupted mass driver, or an overly complicated newton’s cradle.   

       But you still are not going to get any more energy out than you put in. If you drop the first ball in from a distance of two inches from the center of the magnet, the ball on the other end is going to pop out to a distance of two inches from that same center   

       TANSTAFL
Galbinus_Caeli, Sep 14 2006
  

       Momentum seems to be conserved the pack seems to recoil.   

       Energy seems to be conserved. The ball bearing entering has a small amount of kinetic energy and lots of potental energy due to attraction from the magnet. The ball bearing leaving is further from the magnet and therefore not going to be have to come out of such a deep potential well.   

       Assuming all the ball bearings are the same, the factors affecting a compund weapon of this type would be the limits of elastic collisions of the ballbearings and the magnets. As soon as either starts deforming during a collision energy would be lost. Imagine repeatedly dropping a ball bearing onto a metal surface from increasing heights. Imagine the impact speed that starts to damage the ball bearing (or the magnet) -- that's the speed at which the efficiency of the compound magnet gun would start tailing off.   

       Finally, all this is under the assumption that all the ball bearings are identical. If I were building a toy like the one in the linked video, the ballbearing that gets fired would be very different to the rest. It would be non-magnetic to reduce the retarding force of the magnet on it and it would be much lighter to make the momentum transfer more visible. These changes would invalidate this toy's use as a compound weapon.
st3f, Sep 14 2006
  

       Baked (link)."This is simply a wooden ruler with 4 super magnets taped along the length. Ball bearings are set between them so that the introduction of a new ball bearing at the start (top) causes a rifle-like chain reaction of force that pushes the red ball off at the end"   

       Theirs has only two ball bearings after each magnet, and, yes, the last one is non-magnetic.
spidermother, Sep 14 2006
  

       All but the last ball bearings on the far side of each magnet should be nonmagnetic. Otherwise the magnetism will be transmitted along the line of ball bearings the the one which is supposed to move forward into the next magnet. It would be preferable if the others were carved out of petrified whale testicle.
bungston, Sep 14 2006
  

       //find a novel use for it// maybe a defence system built into a tall tower? suspend the whole thing vertically so that gravitational potential is added to the magnetic. im imagining a fort with these built into the towers. sevreal in each tower, so that it can spray whatever chosen ammo out in all (outward) directions. Given, it would need to be a really tall tower, but might work, if its shoting poison darts or smoke bombs, someting relatively light.   

       [st3f]: hadnt seen the video, and still cant since my players are outdated and im lazy, but i saw a picture similar to the one in [spidermother]'s link but only a one stage. thanks to both, i couldnt find again. the idea was to chain a copious ammount of these together, and space them so that they have the same sort (maybe not magnitude) of acceleration as an active railgun. I'm not looking to achieve orbit with one of these, just shoot someting a fair distance with nearly no energy input (except setup/reloading).   

       [gc] what is TANSTAFL?
bleh, Sep 15 2006
  

       TANSTAAFL=There Ain't No Such Thing As A Free Lunch (or in your case, Launch).   

       Oh, and as a weapon, this is going to have a ridiculously long and complex reload cycle. Each ball bearing is going to have to be moved back to its starting position. Forget just one, and you have a misfire.
Galbinus_Caeli, Sep 15 2006
  

       Considering the time it would take to set for each shot, I don't think using such a device would be practical in a serious offensive role.   

       Ah, sorry [G_C] you beat me to it.
zen_tom, Sep 15 2006
  

       //I don't think using such a device would be practical in a serious offensive role.//   

       I doubt it would work offensively also, but defensively in a fort, maybe, im sure it would be a lot less efficent and complicated than existing systems, but i think it would be more interesting. also, depending on the strength of the magnets and the ammount of bals you could suspend from each, it may be possible to fire several times before reloading, then the incredibly long cumbersome contraption could theoretically be fired in reverse.
bleh, Sep 15 2006
  

       To phrase it differently: The magnets are attracting the stacks of bearings, which would balance best with an equal number on each side of the magnet. So the bearings are moving "downhill", magnetically speaking, as the gun fires. The stacks of bearing move closer to the equilibrium of equal numbers on both sides of each magnet.   

       That's where the power comes from, and where you could calculate how much energy is available. This is only good for one shot, (maybe another a lot weaker) so it isn't perpetual motion. Recocking will take energy, as the bearings will have to be lifted "uphill" away from the magnets.   

       [Later] This isn't an original idea. Fishbone.
baconbrain, Sep 15 2006
  

       The way this is described, it will effectively act like a magnetic "crossbow". Is it possible to actually get a higher projectile velocity with this than a system based on springs? Better novelty value, anyway.
NoOneYouKnow, Sep 16 2006
  

       I was thinking instead of having regular magnets what about electro magnets so then with the origanal idea- when the ammo goes down the track it gets pulled and then that magnet shuts off before the ammo gets stuck to it and that just keeps happening, this is actually a gauss cannon/gun/whatever and not a rail/cannon/gun/whatever. but its still pretty cool
Blue Wizard, Nov 26 2006
  
      
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