Why does a spherical magnet fall so slowly through an aluminum tube?


Juggalos, obviously.

It was Eddy somebody.


It’s an interesting effect, and it makes me wonder if there has been any practical application of it. Frictionless brakes of some sort?

Aside: I agree with the science teacher’s conclusion that there must be a divine being who made ice float, but I disagree with his reasoning. Rather than being “rigged” so that ice doesn’t freeze on the bottom and thereby kill fish in ponds, thus allowing for life on planet earth, I believe that the divine being created ice that floats to maximize the enjoyment of soda, which would be a very different experience if ice sunk to the bottom; sinking ice would cool less efficiently, and it would actually increase in volume as it melted, thereby overflowing a glass. The commonly known principle of “nature abhors a warm Coke” is obviously divine, so clearly FSM wants you to enjoy a cold beverage with your spaghetti dinner.

I, Pastafari.


Eddy current brake.

Used for applications where braking force is a safety issue, like roller coasters. As long as basic mechanical maintenance is done, these brakes will not fail. Also used in the auto belay machine at the climbing wall.


I have a copper tube and some disk magnets on my desk that I play with on occasion. I don’t know why it’s suggested only spherical magnets exhibit this phenomenon.

The only advantage i can see to using a spherical magnet in this demonstration is that it is less likely to get stuck in the tube. I’ve found that if a small enough magnet is put through and it makes contact with the side it will come to a complete stop.


This question is borne of my incomplete (probably high-school level, at best) understanding of the forces at work. Your statement “its interaction with electrons in the tube requires energy, and the energy is obtained by stealing it from the pull of gravity” prompts me to ask - what happens when the experiment is moved to a place free of gravity? Obviously, letting it ‘fall’ down the tube wouldn’t be possible, but you could throw/fire it down a tube to get the motion started.

I had a second question too, but I’ll be darned if I can remember what it was…

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These guys?

Right, and in that case the energy would be “stolen” from a different force - inertia.

The second video really makes this effect much more clear to see - from the first video, I totally thought “it looks like friction is slowing down the magnet” - but you can see it much more clearly when the ball travels from PVC to aluminum. Neat!

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If the statement is valid (probably more like stealing from the pull of gravity working on the magnet), then I guess you can just substitute momentum for gravity.

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I give those guys a right-handed thumbs-up!


Hah, eddy currents are so much mumbo-jumbo. I’ve never seen an eddy current. I think it’s the power of Alumination myself.

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It’s the Aluminati!


This is a totally cool trick that I want to use to impress my five year old now. :slight_smile: Daddy is magic!

This effect is used for a different purpose in the AC induction motor.

In that motor, which is an inside-out version of the experiment shown here, the magnet is a cylindrical electromagnet called the stator, driven by alternating current. The aluminum is formed into the shape of two rings, one at each end of the spinning part of the motor called the armature, with aluminum wires connecting the two ends. (The aluminum is cast into a steel-laminate rotor assembly to form the rings and wires, to save assembly cost.)

The armature has a magnetic field induced into it by the alternating current applied to the stator winding. This magnetic field then interacts with the stator field to cause the rotor to make torque. Except instead of making a magnet fall slowly, it turns a load. This is how nearly everything in industry is moved.


I guess I wasn’t thinking of it that way. I know how induction motors work, of course. I was more curious about uses of it as a braking or slowing force through a tube. The @sethk video reminds me of this idea I had as a kid for creating some kind of fire escape tube that would get people out quickly and safely even in the event of a power-outage. Of course, that would require wearing a very large suit of magnets, I’m guessing, so maybe not that exact idea, but something like that.

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A friend, physicist and former co-worker used to argue that whatever divine being created the universe wanted nuclear war to be possible, because if the mass of an electron was only 1% higher, nuclear fusion wouldn’t be possible. Used to have interesting conversations about what life would (or wouldn’t) be like if various fundamental universal constants were shifted slightly. In fact, he wrote a book…


I think this line could be misinterpreted. It would be better stated as “stealing it from the motion of the ball due to gravity.” This way you would not have any confusion that it was “stealing from gravity,” but instead it would be clear that it was stealing from the motion of the ball, whether or not there was gravity.

How can it steal from the motion of the ball? All moving objects (with mass) have energy, specifically kinetic energy (KE = ½ mv2). Therefore, you can always convert the energy from a moving object into some other form of energy.

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Man, Wikipedia is all dick-pics.