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There are several videos on youtube describing linear accelerator built solely from permanent magnets, put like this:

enter image description here

I find it hard to believe that this can work because if it did, it would be exploitable to gain energy. Where would that energy come from then?

I think it's faked and that the magnet should find equilibrium in the middle of the contraption. Am I right or wrong?

I'm willing to test it if I don't get conclusive answer, but I thought I'd as first before wasting money on bunch of magnets.

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  • $\begingroup$ "would be exploitable to gain energy" at the expense of the magnets losing magnetization, because there is potential energy sthttps://physics.stackexchange.com/questions/67107/from-where-do-the-permanent-magnets-get-energy-fromored in the macroscopic permanent magnet. $\endgroup$ – anna v Jan 18 '18 at 10:05
  • $\begingroup$ @annav That's what I meant. Harvesting energy stored in the arrangement of atoms that creates magnetic field - while increasing it's entropy. I wonder what would happen if you tried the same with superconductive magnet with persistent switch. $\endgroup$ – Tomáš Zato - Reinstate Monica Jan 18 '18 at 10:10
  • $\begingroup$ correct link physics.stackexchange.com/questions/67107/from-whe‌​re-do-the-permanent-‌​magnets-get-energy-f‌​rom stored in the macroscopic permanent magnet. You cannot win, the energy to make superconducting magnets is much more than you would get out. $\endgroup$ – anna v Jan 18 '18 at 10:18
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In the video I see that the author needs quite some force to load the gun. So here he puts the projectile in a high potential state, i.e. he provides the energy during the loading process. When released this energy accelerates the projectile. Hence, no violation of energy conservation.

Edit Here a small representation with python and ten dipole magnets: enter image description here Top: field lines in top view. Bottom: x-Field (arb.u.) on projectile path.

If you came from the left you have to somewhat overcome the negative bump. You see that the last max is almost as high as the first min is deep. That's why lubricant is important, as metnioned by @xcoderx. Actually, the thing looks better if you make it shorter. On longer ones the max approaches a saturation value and you'll have more friction loss.

Extra Just looked at the video again because the author inserts the magnet as [NS] and [SN]. The behavior corresponds to the lower graph. In one case one has to overcome the first bump by pushing, but the total distance is not very much as the magnet is later attracted by the second bump. In the second case, the reversed magnet is attracted by the first bump, the author then pushes it over the second bump, which the naturally results in a larger distance.

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  • $\begingroup$ So it's kinda like magnetic slingshot? $\endgroup$ – Tomáš Zato - Reinstate Monica Jan 15 '18 at 12:17
  • $\begingroup$ Well, that would probably the most simple case: just press two north poles together, then release them. The nice thing here is the field geometry that allows for a "maximum" in the 1D path, so you can load from one side and it is released when you overcome that "max". Note that in 3D there is no maximum but a saddle point, due to properties of harmonic potentials. $\endgroup$ – mikuszefski Jan 15 '18 at 12:22
  • $\begingroup$ But what if that contraption was longer? Shouln't there be a limit on it's length? Also I assume it wouldn't work in a circle, right? $\endgroup$ – Tomáš Zato - Reinstate Monica Jan 15 '18 at 12:23

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