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String-free Newton's Cradle

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This would only work in deep space, where there are no significant perturbations from the gravitational pulls of other bodies.

A series of four identical rigid, elastic balls (depleted uranium might be nice) are first arranged in a row, just touching one another. A fifth, identical ball is then placed some distance from these four, but on the same line. Gravity and the elasticity of the collisions will then do the rest. The period of oscillation of the system will depend on the mass of the balls but, for any reasonable mass, will be quite considerable.

The long-term stability of this arrangement will depend on the perfectness of the balls and on the precision of their initial alignment; any imperfections will tend, over time, to cause the arrangement to become unstable. However, a pragmatic engineer might arrange for the contact points to be very slightly flattened (so that contact is made over a disc-shaped area, rather than at a point).

MaxwellBuchanan, Mar 20 2012

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       [+] this should be our envoy to other worlds.
FlyingToaster, Mar 20 2012
  

       [+]. Because there are no strings it couldn't properly be called a 'cradle'. I would name it in honour of its inventor but as "Bucky Balls" is already taken and "Max Balls" sounds, well slightly salacious, I'll stick with "Newton's Balls". Of course, in space no one can see your strings.
AusCan531, Mar 20 2012
  

       [+] "We send this remarkable executive desk toy as an example of our staggering engineering capabilities, and hope that our two worlds can peacefully initiate economic coitus. Please read operator's manual before use. Some assembly required. Hecho en Mexico."
Alterother, Mar 20 2012
  

       This is clearly what the Iranians are secretly building.
xenzag, Mar 20 2012
  

       Newton's Five Body Problem ?
AbsintheWithoutLeave, Mar 20 2012
  

       Hmm, 2 taking elements from the idea, namely; Uranium and Newton's Cradle, it occurs to me that it might be nifty to use a Newton's Cradle arrangement to manage the core of a nuclear reactor. Take 5 equal masses that together form a critical mass, and have them arranged into a Newton's Cradle. Only for instances when all 5 are in contact would the chain reaction occur, and as long as you could keep the cradle swinging, you'd be able to run a (relatively) low-intensity reactor, controlling the frequency of critical instants by lengthening and shortening the strings.
zen_tom, Mar 20 2012
  

       Lovely idea - however what, I wonder, is the probability that this, or an approximation to it, already exists? Let's say that there are a hundred billion small rocks floating in space in our solar system, a hundred billion similar stars in our galaxy and a hundred billion galaxies in the universe. This suggests that there are about 10^33 small rocks floating about - surely somewhere there must be five rocks arranged in a line?
hippo, Mar 20 2012
  

       //However, a pragmatic engineer might arrange for the contact points to be very slightly flattened (so that contact is made over a disc-shaped area, rather than at a point).//   

       This will happen anyway - the spheres will deform momentarily at the point of impact - this is how the force is transferred between them.   

       To keep them aligned, I wonder if you could put a charge on them and use some kind of scaled up variant of a Penning trap.
Wrongfellow, Mar 20 2012
  

       ^ magnetism perhaps ?
FlyingToaster, Mar 20 2012
  

       //the spheres will deform momentarily at the point of impact//   

       True, but the deformation is what provides the springing force. In other words, if the spheres are even slightly imperfect (or imperfectly aligned), the compression and rebound will throw them progressively further "off" on each bounce. Small flats machined onto the spheres would prevent this.   

       Imagine dropping a bouncy ball onto (a) a flat floor and (b) another bouncy ball. In (a), the ball will bounce vertically; in (b), it will only do so if it is aligned to hit the top dead centre of the other ball.   

       Perhaps this could be run as a competition between the advanced nations of the galaxy, to see who could machine and align the balls accurately enough to work for many bounces.
MaxwellBuchanan, Mar 20 2012
  

       I'm not sure a flat portion would work, but maybe a cunningly shaped impact point could be designed, so that if the spheres hit each other slightly off-centre, the reaction force tends to restore the alignment rather than degrading it?
Wrongfellow, Mar 20 2012
  

       I think to work that the balls would have to be incredibly massive (even without any perturbations from anything else). Gravity is an incredibly weak force, and likely that if displaced, the 5th ball would sit at a distance for all eternity (or at least until the second law of thermodynamics has reduced the universe to an entropically even, feezing grey soup).
Suggest carving them out of a neutron star rather than using uranium. Impractical? Of course, but not more so than setting up five balls of uranium in deep space...
goff, Mar 20 2012
  

       Would two lumps of neutron star rebound elastically, or would they merge? I'd suspect the latter.
Wrongfellow, Mar 20 2012
  

       Depleted uranium with a chewy neutronium centre... could that keep the N from uncollasping ?
FlyingToaster, Mar 20 2012
  

       If the neutronium centre was large enough to be stable then wouldn't it just crush the DU into more neutronium?
Wrongfellow, Mar 20 2012
  

       //but maybe a cunningly shaped impact point could be designed// - if the process is slow enough and the balls are sufficiently massive, the civilisations which live on them will have enough time to reshape the impact points between collisions.
hippo, Mar 20 2012
  

       Hmmm...Wrongfellow, you're probably right. If you carved uranium balls big enough to have any gravitational attraction, then they may also be too massive to bounce anyway - just plough into each other and deform and conjoin. It still sounds elegant in theory though.
goff, Mar 20 2012
  

       If you have enough compact stuff to produce 1g at its surface, then you have a shell of DU that is under the horrific stress of... 1g.
FlyingToaster, Mar 20 2012
  

       1g isn't enough to keep degenerate matter stable. If you could somehow magic up a lump like that, it would immediately explode.
Wrongfellow, Mar 20 2012
  

       Yes, it does, MUHWHAHAHA !   

       Ahem.   

       // the spheres will deform momentarily at the point of impact //   

       Uranium is pretty soft ... it will deform considerably and absorb a lot of energy in the process. They will need a tough shell of steel or similar for this to work.   

       Also, the impact velocity of the incoming ball needs to be lower than the escape velocity of the system as a whole, otherwise the ball at the other end of the stack will continue in a straight line, and you'll have to wait for it to go all the way round the Universe and come back from the other side.
8th of 7, Mar 20 2012
  

       //the impact velocity ... lower than the escape velocity//
Well it would be, wouldn't it: that's how you start one of those things.
  

       Think big. A solid Osmium sphere the size of the Moon would have 2/3 Earth's gravity. Of course I'm gonna go out on a limb and say you'd end up with a large pile of Osmium dust rather quickly, so something else: maximum bounce & maximum density.   

       Tetraneutronium n4 should have a decent specific gravity.
FlyingToaster, Mar 20 2012
  

       I was thinking to start with something more modest than a moon, say a metre or ten in diameter. The oscillation period would be quite long...
MaxwellBuchanan, Mar 20 2012
  

       or cheat a bit: nuclear batteries charging internal electromagnets should help keep things lined up, as well as give a bit of an assist to gravity.
FlyingToaster, Mar 20 2012
  

       //I think to work that the balls would have to be incredibly massive//   

       Why? If you want truly elastic collisions, you're going to need to be in the tonnes-or-less scale, otherwise collisions will result in plastic deformation (inelastic collisions), and also I think you'll have problems with wave propagation and complete transfer of energy for large objects.
Custardguts, Mar 21 2012
  

       What [Custardguts] said. They tried a Newton's cradle using wrecking balls on "what would happen if". It didn't really work.   

       Most materials are, at best, close to linear and elastic within a range of stresses, which is a large part of the reason for the invention of theorbos and overwound strings in music. Very gentle collisions, and very large balls, are both problematic, and it's hard to see how you'll avoid both.
spidermother, Mar 21 2012
  

       Yes, the long handle of a theorbo allows slower swings. Nutting someone with a lute tends to damage the bowl, or the soundboard, or both.
pocmloc, Mar 21 2012
  

       //Very gentle collisions, and very large balls, are both problematic,//
You know, Mrs AWOL commented that very same thing only last night.
AbsintheWithoutLeave, Mar 21 2012
  

       That was inevitable.
Alterother, Mar 21 2012
  
      
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