Ask HN: using only static magnetism - impossible to stably levitate against gravity? | Hacker News Reader

Ask HN: using only static magnetism - impossible to stably levitate against gravity?

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3 pointstocomment10 comments

I don't get it? Why would something like below work:<p><pre><code> ________ SSSSSSSS NNNNNNNN </code></pre> NNNNNNNNNNNNNNNNNN SSSSSSSSSSSSSSSSSS<p>Top magnets are glued to a board, a larger area of magnets are glued to the floor. Surely the magnets wouldn't have the strength to flip the board, so how is that not stable?

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Earnshaw’s theorem applies to electrostatic fields (ie, from non-moving charges) as well as magnetic fields. I think electrostatics are a bit easier to visualize than magnetostatics.

There’s also a good analogous physical situation. Imagine you have a rubber sheet stretched into a plane. The challenge is to create a local minimum in the sheet by pushing it up in places. If you push it up with a tube to try to create a bowl, instead you’ll get a flat circle on top of the tube.

My freshman year physics professor had us play with a program that simulated kinetic physics for point charges. You could “glue” charges to a plane, and then release other charges to see how they would respond. The end of the assignment said “Try to create a cage that can hold a charged particle. If you do, you might be eligible for a prize.”

Any idea where I could find that program. I think that would help me. My problem really is visualizing it.

I don't get it? Why would something like below not work:

        ________
        SSSSSSSS
        NNNNNNNN

    NNNNNNNNNNNNNNNNNNNN
    SSSSSSSSSSSSSSSSSSSS

Top magnets are glued to a board, a larger area of magnets are glued to the floor. This would be in two dimensions. Surely the magnets wouldn't have the strength to flip the board, so how is that not stable?

IANA physicist, but how could magnets be strong enough to lift the board off the ground but not strong enough to flip it?

I thought of that, but what about magnets barely strong enough to lift the board, maybe 1cm or levitation or something?

Then wouldn't the board start to destabilize until an edge hit the ground? So it wouldn't be floating exactly, just sort of eerily resting.

True. Good point. What if you curved the bottom piece into a slight dish shape?

Lateral stability; the board will slide until it reaches the edge and then it will fall off or flip over.

Unless you have some sort of lip to catch the suspended part, it's not going to be stable.

Or you could use a tension member to hold it in place, although that breaks the aesthetic slightly.

I may just have to try it huh? Where can I buy a whole lot of cheap magnets?

For extra credit, explain why the Levitron DOES work:

http://www.youtube.com/watch?v=05FDGCis-Sw

(no camera tricks in that BTW, it's just a magnet top and a magnet bottom. The weight in the top must be carefully adjusted in order for the device to work. It will continue levitating for as long as the top keeps spinning fast enough - if you carefully blow on the top's side to speed it up, you can keep it levitating for many minutes.)

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