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Can Repelling Magnets Replace the Spring in a Pogo Stick?
- rrggrr 8y agoFully compressed I believe the opposite poles fields begin to attract each other, counteracting the matching poles repelling one another, and explaining the alternating pull force.
- jaytaylor 8y agoThis is easily solved by including a minimal height spacer between the two magnets to prevent them from getting too close.
- dsfyu404ed 8y agoTL;DR Yes but not very well. A spring made from magnets doesn't have a close to linear spring rate like a traditional coil spring does.
- jtms 8y agoCoil spring + magnets = success?
- dsfyu404ed 8y agoYou're fundamentally going to be limited by the fact that magnetic force is roughly exponential with regard to distance whereas coil springs are roughly linear. By stacking a bunch of magnets in the article they reduce the travel of any one magnet pair so it's closer to linear. If you look at short enough segment of a graph of an exponential equation it will look linear, this is basically what they're doing.
- stiglitz 8y agoYou mean quadratic, not exponential.
- bzbarsky 8y agoActually, the answer is probably "it's complicated". https://en.wikipedia.org/wiki/Force_between_magnets#Force_between_two_bar_magnets https://en.wikipedia.org/wiki/Force_between_magnets#Force_be... gives an approximation in the x >> R case, but in the pogo stick situation we're not in that case, especially once we get to 5 magnets or so. But even there, if x >> L also holds (which it would in this case, since the magnets shown have R and L of about the same size), the force goes as 1/x^3 or so, because the 1/x^2 terms cancel out if you go through the small-L approximation. Which, again, is not relevant to the problem because our case has small x. For small separations, the exact geometry of the magnets starts to matter. http://www.magnetsales.com/design/faqs_frames/FAQs_2.htm http://www.magnetsales.com/design/faqs_frames/FAQs_2.htm has some experimental results that show the field going up a bit over 3x when distance drops from 1 to 0.5, then ~2x when going to 0.25, then ~1.5x when going to 0.125 and only changingby 1/8 when going to 0.063... all distances in inches from _surface_ for a magnet that's radius 0.5 and length 0.5 in the same units. Note that this means that the distances from the _center_ are approaching 0.5. All that said, the force is almost certainly not exponential, I would guess.
- jldugger 8y agoBut is it linear, from the data? They had two datapoints and projected from that sparse data a regression line.
- twtw 8y agoThere are far more than two data points on all of the charts in the article. What chart are you looking at?
- jldugger 8y agoThe youtube video mostly.
- mindslight 8y agoNo, it's linear from basic physics.
- mhh__ 8y agoWithin a certain operating region (stress and strain, i.e. the straight bit on the stress/strain graph), a spring will behave linearly e.g. F = kx. In real life it should be pretty close to linear, but it would be worth checking as I could imagine there being non-linearity or time dependence of a spring under what is probably a decent amount of load being applied quickly and repeatedly.
- thomasahle 8y agoYes, that would have been fair. The electromagnets are also (close to) linear if you look at small intervals.
- halestock 8y agoHook's law[1] states that the force of a spring scales linearly with the distance it is stretched. [1] https://en.wikipedia.org/wiki/Hooke%27s_law https://en.wikipedia.org/wiki/Hooke%27s_law
- cagenut 8y agocould some kind of conical shape offset the different power curve?
- axaxs 8y agoLoved the video. Didn't love the loud beep near the end, that's at about 3x the volume of the speaker the whole video...
- CamperBob2 8y agoYouTube desperately needs an audio compressor feature. Can't imagine why they haven't implemented that yet, it's more or less trivial.
- callalex 8y agoI agree that they should, but it’s pretty reductionist and arrogant to call it “trivial”. There is no such thing as a one-size-fits-all dynamic compressor. You would also need a fairly sophisticated content detection system (don’t distort anything that is music, which is a huge use case on YouTube for example). Not to mention adding _any_ steps to their intake pipeline requires more processing power and data moving than you and I will ever touch in our lifetimes.
- CamperBob2 8y agoSpeaking as a reasonably-experienced audio and DSP programmer -- and considering that the feature has existed in just about every home theater receiver for the past 20 years -- yes, it's trivial. YouTube has the advantage of being able to preprocess the audio track in its entirety, although most implementations work on a block basis and don't need that. A 'night mode' feature would take about one weekend's worth of work from start to finish. You would also need a fairly sophisticated content detection system (don’t distort anything that is music, which is a huge use case on YouTube for example). No, you need a button that the user can press to turn the feature on and off. It should default to 'off.' For extra points, make it a slider. Not to mention adding _any_ steps to their intake pipeline requires more processing power and data moving than you and I will ever touch in our lifetimes. Compared to the work needed to process the video itself, it would be a total non-issue.
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- umvi 8y agoSeems like electromagnets could help compensate for distance by drawing more current when far apart, and less when close together. A microcontroller could essentially make the force linear like a spring. Of course, then you need wires hooked up to your pogo, and that might be scary if the microcontroller has a bug (or somehow fails) since you are now essentially hopping onto a railgun.
- jtms 8y agoThis is the best comment I have ever read
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- azhenley 8y agoAbout as scary as my Tesla on autopilot going around a curve at 75mph. Every time I ask myself, what if there is a bug or if it turns off...
- randall 8y agoThe downside risk is never worth it for me. I'm always hands on, and curves I treat it like a drivers' ed situation: You better do the right thing.
- cheald 8y agoYeah, I tell people that I treat it like a well-mannered 15-year-old driver - I expect it'll do the right thing, but I'm here to take over in case it finds itself out of its depth.
- weberc2 8y agoPretty much every critical system in a vanilla Toyota is already software-controlled, not to mention the legion of critical systems involved in air travel, elevators, medical equipment (e.g., pacemakers), etc. Not sure that this should bring comfort, but at least you need not be particularly frightened by your Tesla. :)
- amelius 8y agoThey should try this with electromagnets, and make the curve linear using electronics (and perhaps find the optimum curve in terms of fun when jumping around).
- emilfihlman 8y agoThe core issue that's not mentioned is that we want a certain kind of a curve for deceleration because that what determines how the spring works. Little resistance first and lots at the end means it's not much different from just falling down!
- 8note 8y agopermanent magnets tend to demagnetize when you beat them around, too
- rootusrootus 8y agoCould you use a lever to multiply travel distance while working with very strong magnets close together to make the force curve a bit more linear?
- 0ld 8y agothey did freedom units math! quite a feat
- v768 8y agoYep, still quite hard to read.
- Xcelerate 8y agoI’ve often thought it might be interesting to replace the springs in a piano’s action with electromagnets so you could have control over the exact touch profile.
- mikepurvis 8y agoI don't have a full text link, but definitely there's been some work done on at least the front end of this problem: https://uwaterloo.ca/motion-research-group/publications/kinematic-model-piano-action-mechanism https://uwaterloo.ca/motion-research-group/publications/kine...
- tomcam 8y agoOr you could pay a piano tech a couple hundred, then have nothing to worry about for the next few generations
- callalex 8y agoI don’t think pianos typically have any springs, everything is gravity powered with counterweights. The only springiness comes from striking the strings as far as I know. Only the really cheap electronic keyboards use springs. Even halfway decent electronic pianos still rely on counterweights.
- Xcelerate 8y agoI’m pretty sure even really nice uprights (Yamaha U3) have jack springs... maybe I saw the wrong diagram
- ordu 8y agoI cannot speak about any piano, because I had chance to tinker with only one, but that piano had springs. Keys itself return into their normal position because of gravity, but part of key action mechanism was powered by a spring. I discovered it while trying to fix some keys which was too slow to "recharge", to return to a normal position after a soft note, so I was unable to make a sequence of a short duration soft notes from the same key. I fixed it by stretching springs long enough for a plastic deformation to occur. Loud notes worked ok, because their "recharge" was driven fully by springness of the strings. Though it was just one piano, I do not know how different the mechanics can be between different pianos.
- jcoffland 8y agoI would still like to see someone build a pogo stick with many magnets with alternating polarity. This article indicates that it would work. Seven strong magnets with a 1" initial gap between each of them should work.
- hammock 8y agoCan we make an electronagnetic-sprung mattress?
- J5892 8y agoThat would really suck in a power outage.
- mhh__ 8y agoOr if you wet yourself
- diabeetusman 8y agoIgnoring other problems mentioned, what would the benefit of such a system be? I suppose it'd be very customizable, but probably expensive (both initial cost plus the cost of the electricity to run it).
- hammock 8y agoI guess I was reading the other comments and thinking eminently customizable springiness - to the square-inch resolution. People pay outrageous amounts for Sleep Number beds. This would be the high-end version of that.
- madengr 8y agoWhere does the heat go? The spring will have a much larger surface area to dissipate the heat. Maybe surrounding the magnets with a copper tube will conduct some eddy currents.
- notacoward 8y agoA lot of science museums have a display showing how magnets and copper pipes interact. In short, because of Lenz's Law, the magnet's motion induces an opposing magnetic field in the pipe ... which suggests another possible way to make a pogo stick with the right kind of resistance curve as well as addressing the heat issue.
- rubatuga 8y agoThis means the kinetic energy is lost to heat, which is not good for a pogo stick
- madengr 8y agoYes, but there will always be some heat loss, and the magnetism is lost with heat. I’m just assuming the pogo stick will drastically fail when the magnets get hot enough.
- contravariant 8y agoA mechanism that absorbs any kinetic energy would make for a very disappointing Pogo stick.
- notacoward 8y agoIf it absorbed all of the energy that would be a problem, but any child who has seen one of those science-museum displays I mentioned can tell that's not the case.
- contravariant 8y ago
- dchichkov 8y agoSeem like an attempt of replacing a large, fine-tuned, atomic scale electromagnetically interacting system with a much simpler, less fine-tuned one.
- sitkack 8y agoThe Universe is already running the best code for the job.
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- truethrowa 8y agoHow can they stack three magnets with the repelling side facing each other on all three magnets? Isn't one side repelling and the other attracting?
- shagie 8y agoYou can do some interesting things with magnets. Consider the Halbach array ( https://en.wikipedia.org/wiki/Halbach_array https://en.wikipedia.org/wiki/Halbach_array ) often used for refrigerator magnets. That said, the 3 magnet configuration would be: NS - SN - NS From the article: > We added a third magnet, alternating which way the polarity faces with each magnet. Each magnet repels any adjacent magnets.
- Sharlin 8y ago> For the 4, 5 and 6 magnet stacks, the outermost gaps tended to be a bit larger than the inner gaps. Why? > As we add more magnets to the stack, the maximum pull force alternates up and down. If 2 magnets repel with some force, 3 is a little less, but 4 is a little more 3, but 5 is a little less than 4, etc. Why is this so? Shouldn't these guys have figured it out pretty easily? Both seem to be readily explained by the fact that it's not just the adjacent pairs of magnets that interact. Given magnets ABCD, the A magnet is repelled by B and D but attracted by C. B is equally repelled by A and C, but attracted by D. And so on.
- hinkley 8y agoSeems like they missed a trick by arranging the magnets above three so that some of them are paired. You’d probably get more travel but not sure how the force would be affected and neither did they. You could try A BC D, AB CD, AB C DE and A BCD E.
- debt 8y agoIn the similar vein I wonder if exploding lithium ion batteries can be used as small rocket fuel
- CapacitorSet 8y agoUnsurprisingly, an inverse-square mechanism can't replace a linear one over a wide working range.
- bigiain 8y agoI wonder if there’s a Fourier transform equivalent that’d let you stack up various inverse square curves to appropriate a linear curve?
- ta_egdhs 8y agoyes. any set of complete functions form a vector space and can be used in a linear sum to appropriate other functions in the same space. the dot product tells you the value of the coefficients. The fourier transform is just a special case of this.
- beefman 8y agoInverse-cube for magnets
- tedunangst 8y agoThe response could be a closer approximation of linear by using six tubes, two magnets each, with varying initial distances. As the stick compresses, additional magnets contribute force, but not all at once.
- scythe 8y agoJust have two fixed magnets and two moving ones: [+- +-> ===== <-+ -+] As long as the open-wide distance is capped you should have a smoother distance-energy diagram.
- stevespang 8y agoThe ability of permanent ferromagnets to resist (or survive) attacks on their permanent field orientations is related to their intrinsic coercivity Hci. In general, rare earth magnets are much more resistant, however, radiation, excessive heat, serious EM pulses, extreme magnetic field pressure as in this case, will degrade magnets. The degree of symmetry is relevant, as highly asymmetrical designs are much less resistant to magnetic field pressure such as in the case of Halbach arrays. I have constructed several Halbach arrays from N35 Neo magnets purchased from K&J magnetics 10 years ago that have lost approx 50% of their force compared to when newly built. Magnet manufacturers can produce a custom run of a high, extremely high, or even ultra high Hci magnets for a price. In past times this was done by increasing expensive dysprosium content, but now there are multiple ways, and I would suggest the Japanese lead in these efforts. I believe Ames Labs patented a form of nitrogen doped carbon which has an Hci that is rather astounding, but to date it is not a stable product.
- xvedejas 8y agoWould it be possible to put the magnets in some kind of tube which blocks magnetic flux, in order to get a linear relationship instead of the square law?
- baroffoos 8y agoDon't magnets lose their magnetism with repeated drops?
- briantakita 8y agoMaybe a series of magnet pairs, spaced at different heights, could approximate the spring effect. Each pair would be bound together (e.g. with strapping) with the negative charge being on the outside, since Earth has a negative charge. The bottom of the stick would have the strongest magnet, negative charge facing up. - + + - The pairs could then repulse each other at a closer distance, approximating the spring function. I accept royalty donations :-)...but it's probably patented by now.
- dTal 8y agoIf you really wanted to do this, I reckon the way would be to have one set of magnets on the moving shaft and another set surrounding it, so the the repulsion tends to keep the shaft centered. Then you make the shaft surround taper towards the top, so that the further the pole is pushed in, the closer together the magnets are forced. You should be able to achieve any force curve desired with this setup by making the taper non-linear.
- harshreality 8y agoJust use current designs which, I would think, mechanically constrain the pogo stick except in one direction. Replace spring with powerful magnet (pair), and make sure there's a stop pin or something similar to keep the magnetic pogo stick from disassembling itself. I think you're making things too difficult and expensive if you're trying to magnetically suspend and stabilize the pogo stick in multiple/all directions. Also, magnetic radial stabilization would be stable only in a rough sense. The pogo stick handle would still wobble and be disturbingly/unexpectedly unsteady. I think for pogo sticks you'd really want a mechanically confined, piston-like design. You could still have magnetic stops on both extremes.
- dTal 8y agoAlthough it's an interesting idea, I didn't mean to suggest only using the magnets to center it - I was merely clarifying the geometry. Another advantage of my design is that it allows (requires, in fact) one to use many small magnets in place of a few large ones, which is safer and easier to engineer. But as for expense - I think that once you decide to replace springs with magnets in a pogo stick, you have left the path of economy anyway and might as well go for broke.
- jcriddle4 8y agoWhat about a hybrid system with springs for the longer travel but magnets for when you are about to bottom out?
- EADGBE 8y agoI literally can't wait for N-Gate's distillation of this thread.