Fun with neodymium magnets

Neodymium magnets are extremely strong, as anyone who has ever handled them knows. This video shows how strong the forces generated by them are. One should be very careful though, because you can hurt yourself and any nearby electronic equipment with these magnets.

One of the practical uses of these magnets is to have cows swallow them. Yes, really. This is because cows often swallow bits of metal (nails bits of wore, staples, etc.) that are in the pastures where they feed and this can harm them. These powerful ‘cow magnets’ settle in a part of the digestive system called the rumen, where they collect the metal pieces that pass by and prevent them from going further into the system and causing harm.


  1. Ice Swimmer says

    AFAIK neodymium magnets can crack if they are allowed to collide with iron/steel structures, the field is so strong.

    Also, apart from the fact that magnetic fields from the many small spherical magnets counteract the force, also as the neodymium-based magnetic material has a rather low permeability (only slightly higher than air) , so it doesn’t concentrate the field like iron would do.

  2. flex says

    Before they used neodymium cow magnets, they used ferrite cow magnets. They’ve been around a long time.

  3. Ridana says

    So his caveat about steel instead of magnets made me want him to put a slab of meat on the brick and then drop steel pellets on it, or a chunk of steel. Am I evil for wanting that?

    The slo-mo was pretty cool, like a swarm of beetles attacking.

  4. Ridana says

    In rewatching the slo-mo, it’s interesting to me how fast they disperse over the brick, so fast that they’ve already distributed themselves before the force of their initial impact can move the brick. I’m having trouble understanding the physics of that (not being a physicist plus hating math). Like where is the force being stored during the delay between impact and reaction to the impact, while the pellets are still dispersing? Is it the magnetic field of the pellets on the far side pulling the brick, rather than (or in addition to) its movement being a result of the impact? Same question when they’re dropped from above, and the delay before the brick lifts off the surface.

  5. Mano Singham says


    The force is not being stored anywhere. Each of the tiny magnets and the large magnet are exerting forces on every other magnet. As they are brought together, these forces are constantly changing and cause each one to move in its own way. The small magnets ones closest to the big magnet get stripped away from the rest first and attach themselves to the big magnet.

  6. Rob Grigjanis says

    John @7: Work isn’t done by a displacement. If an object is displaced a distance d by a force F due to some field (electric, magnetic, gravitational, etc), the work done (assuming the displacement and force are parallel, and neglecting friction) by the field is Fd*.

    *If the force varies over the displacement it’s a bit more complicated, involving an integral.

  7. Ridana says

    re 5) Thanks, but that’s not quite what I was asking (not blaming you for not being able to decipher me!). Why is there a delay between when the pellets hit and disperse, and when the brick magnet moves. I assumed it moves from the force of the initial impact, and I would’ve expected it to move before or while the swarming happens, instead of after it was nearly completed, but you’re saying that’s not what moves it?

    If I strike a billiard ball with the cue ball, it rebounds immediately. It doesn’t sit there waiting for its inertia to be overcome until it finally moves. (Or maybe it does? I’ve never watched it in slo-mo. :D)

  8. Rob Grigjanis says

    Ridana @10: If you look at the first impact in slow motion, the brick is initially moving towards the cube of pellets. There are two sets of forces at work as the pellets start to fly. The first is the impact of the pellets as they hit the brick; that tends to slow the brick down. The second is the increasing magnetic pull as the pellets get closer to the brick, tending to pull the brick towards them, just as the brick pulls the pellets. The magnetic force increases dramatically as two magnets near each other. So that could make it look as though the brick isn’t immediately responding to the impact.

  9. Ridana says

    12) Thanks for pointing that out Rob. I thought the forward movement was residual inertia from him shoving the brick toward the pellet stack. After watching it a few more times, I can see that it does actually tilt forward a frame or so before it’s clobbered by the pellets. Ditto for the drop from above (I wonder what that surface is? It bows like a trampoline). That makes sense to me now.

  10. Rob Grigjanis says

    Ridana @13:

    I thought the forward movement was residual inertia from him shoving the brick toward the pellet stack.

    It is. The magnetic fields of the pellets probably don’t have much effect on the brick’s motion until just before they impact the brick*. But it would be a nightmare to try and figure out the relative contributions of friction and pellet impact in slowing it down, and the pellets’ magnetic fields pulling it onwards.

    *Gravitational force goes like inverse square of distance, but for magnetic force between magnets, it’s inverse fourth power of distance, so doesn’t really have much effect until they’re really close.

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