Where does the energy of magnets come from?

[Jurph]

Okay, so the bullet leaves the barrel with an initial velocity vector parallel to the ground, and, unconstrained by the barrel, is now free to be acted on by two forces:

1. air drag, which acts in the direction opposite the velocity vector, and
2. gravity, which acts perpendicular to the initial velocity.

In the first full second of flight, the bullet falls ~4.9m, so its velocity vector is always ever-so-slightly below the horizontal, and drifting farther and farther off horizontal. In that time, air drag (which grows with the square of the velocity) is drastically lowering the horizontal speed, and having a negligible effect on the speed at which the bullet falls.

There’s a big however here though. If you fired the bullet off the top of Mt. Everest, the magnitude of its horiztonal velocity will eventually slow to zero and the magnitude of its vertical velocity will approach the terminal velocity (in the downward direction). Since the terminal velocity is going to be much lower than the muzzle velocity, the velocity curve will never have a positive slope.

As for “going to sleep”, unless there’s some weird gyroscopic effect that causes the bullet’s angular momentum to be transferred into linear force, it’s not going to do anything but increase the bullet’s “beta” (ballistic coefficient) which reduces drag but definitely will not accelerate the bullet by any means.

[trupa]

@scm1001

I think we are getting confused with force and energy. A compressed spring can exert a force forever until the universe ends. No energy is expended or taken in until the spring expands or contracts. Where did the energy stored in the spring comes from? From the person who pushed it in. Where is it stored? Well ultimately if one goes deep enough, in the strained electric fields of the atoms making up the spring.

Two magnets can in theory exert a force on each other forever. Where did the stored energy come from? From the guy who pushed them together (NN alignment) or pulled them apart (NS alignment). Where is the energy stored? In the electron spin repulsion of the atoms.

Note that permanent magnets can lose strength just like a spring can, in the former case from magnetic domains losing alignment. This is a separate effect from above and varies from material to material.

Thank you, SCM, for posting this and scratching that itch of irritation in my brain, not to mention saving me the trouble. Good analogy with the spring.

[Spectre_of_Pithecanthropus]

@Garfield226

I imagine those magnets had lost thier charge NOT because the individual atoms had lost their charge, but because the atoms, for one of the reasons I listed above, weren’t lined up properly. It probably could have been fixed by stroking another magnet across it several times in the same direction.

Another way is to hold it, if it is long enough, so that one end is on a hard surface, and the magnet is pointing towards the North Magnetic Pole. (From Sweden, I’m not sure what direction that is…west by northwest?. You give it a sharp rap with a hammer and you’ll knock those atoms back into line. You can weakly magnetize any piece of iron or steel in this manner. I found this very helpful when I had to insert a very tiny Philips screw into a narrow, awkward place. I couldn’t get the screw positioned without having it fall before I could drive it in. So I followed the above procedure with the screwdriver shaft, and, sure enough, it was now a weak magnet. The tiny screw clung to the end long enough for me to drive it in. Yay, me!