Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Can Repelling Magnets Replace the Spring in a Pogo Stick?

kjmagnetics.com

Post reply on HN
Print view View original

51–60 of 149 posts

Re: Can Repelling Magnets Replace the Spring in a Pogo Stick?

#51

Post by bleomycin »

Retric wrote: post #48
weberc2 wrote: post #28

Earlier quoted context omitted.

Pretty 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. :)

Elevators use simple physical safety systems. Which is why you see headlines like: "Skyscraper elevator plunges 84 floors — and everyone survives" https://www.cbc.ca/news/world/elevator-plunge-chicago-1.4913...

Every link i’ve seen to this story annoys the hell out of me because they don’t explain HOW the elevator eventually stopped. That’s literally all I want to know.

Re: Can Repelling Magnets Replace the Spring in a Pogo Stick?

#52

Post by Sharlin »

> 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.

Re: Can Repelling Magnets Replace the Spring in a Pogo Stick?

#53

Post by bzbarsky »

stiglitz wrote: post #45
dsfyu404ed wrote: post #13

Earlier quoted context omitted.

You'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.

You mean quadratic, not exponential.

Actually, the answer is probably "it's complicated". 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 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.

Re: Can Repelling Magnets Replace the Spring in a Pogo Stick?

#54

Post by damon_c »

azhenley wrote: post #19
umvi wrote: post #6

Seems 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.

About 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...

I have used Tesla autopilot on twisty cliffside roads (my hands hovering an inch away). The question in my mind was, when there is "nothing" there, is that a place it thinks is okay to go? Or is it a place a place where it should absolutely not go? I wish it could just tell me that.

Re: Can Repelling Magnets Replace the Spring in a Pogo Stick?

#55

Post by rubatuga »

notacoward wrote: post #50
madengr wrote: post #42

Where 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.

A 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.

This means the kinetic energy is lost to heat, which is not good for a pogo stick

Re: Can Repelling Magnets Replace the Spring in a Pogo Stick?

#56

Post by contravariant »

notacoward wrote: post #50
madengr wrote: post #42

Where 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.

A 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.

A mechanism that absorbs any kinetic energy would make for a very disappointing Pogo stick.

Re: Can Repelling Magnets Replace the Spring in a Pogo Stick?

#57

Post by notacoward »

contravariant wrote: post #56
notacoward wrote: post #50

Earlier quoted context omitted.

A 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.

A mechanism that absorbs any kinetic energy would make for a very disappointing Pogo stick.

If 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.

Re: Can Repelling Magnets Replace the Spring in a Pogo Stick?

#59

Post by TheOtherHobbes »

halestock wrote: post #25
jldugger wrote: post #17

Earlier quoted context omitted.

But is it linear, from the data? They had two datapoints and projected from that sparse data a regression line.

Hook'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

And magnets repel with an inverse cube law. So there is no simple way to replace a spring with magnets.

You could "linearise" an electromagnet with distance sensors and code, but that's a bit of a Franken-Solution - not least because any delay in the closed loop could make the system unstable.

In this case a couple of minutes of research - or maybe asking a physicist - would have saved a lot of essentially rather pointless tinkering.

I mean - if you enjoy tinkering more than you enjoy producing an elegant solution, then tinker away. But this is noodling around for the sake of it, not solid engineering.

Post reply on HN