4 ms·
Dumb question: why is it hard to make something spin really fast? Simple example: put your frictionless spherical cow on a spinny plate. Make it a very small c
by sfink 2y ago
Dumb question: why is it hard to make something spin really fast?
Simple example: put your frictionless spherical cow on a spinny plate. Make it a very small cow; it's only there to have a point of rotation. Why frictionless? You don't want its butt to catch fire. Why spherical? It'll need to maximize volume dedicated to arm muscles; see below.
Have the cow hold two ropes, each leading to a full-sized cow 10m away. Apply force to those cows (blow on them, or magnetize them and do a solenoid thing, or just make them very gassy cows and orient their spherical butts in opposite directions). Get them spinning at 1Hz. (This is very fast; remember the diameter is 20m.) Now have the middle cow pull the ropes, shortening them to 10cm. It's now spinning at 1Khz. 10mm gives 1Mhz. Conservation of angular momentum, baby.
Do this in a vacuum in microgravity, and you don't need the center cow.
Sure, if you're doing this at a bovine scale, the tension is ridiculously large. What makes it infeasible at a small scale?
- mjewkes 2y agoCentrifugal forces on the spinning object will tear it apart once it goes “really fast”
- Thorondor 2y agoThe main issue is that when objects spin really, really fast, they tend to explode due to centrifugal force. The tensile stress on a spinning round, homogeneous object is p * r^2 * w^2, where p is density, r is radius, and w is angular velocity. Using your numbers for a steel cylinder with density 8 g/cm^3 gives a tensile stress of (8 g/cm^3) * (5 mm)^2 * (2pi*1 MHz)^2 = about 8 TPa which vastly exceeds the tensile strength of steel or any other known material. Using cows connected by ropes would be even worse because the enormous centrifugal force would be borne by only a small rope.
- dexwiz 2y agoThere are some great examples on youtube of people demonstrating this with skateboard wheels and compressed air.
- sfink 2y agoThat makes sense. With (strong) steel maxxing out at around 1Gpa, that's 4 orders of magnitude less. The rotation frequency gets squared, so... 10Khz? Huh: "The fastest rotation achievable by standard motors is of the order of 10 kHz". (How does a "standard motor" achieve 10 KHz without ripping apart, then? I don't think many standard motors are 5mm or smaller.) Thinking about this makes me imagine a potter's wheel for shaping a ductile metal. It spins really fast, but you can only reshape in the outwards direction, and the resistance goes up dramatically towards the center. Oh, and if anything flakes off, you're dead. But before you die, you could probably make some pretty artwork.
- jmb99 2y agoI’m not an expert, but I don’t believe there are that many motors that spin at 600k RPM (= 10kHz), and definitely no large ones. That is absurdly fast. Modern turbochargers are some of the fastest-spinning off-the-shelf things your average person can buy, and they normally top out below 200k RPM (and only the smallest/lightest turbines can spin at 200k RPM). If there does exist a 500k+ RPM electric motor, I would be surprised if it was larger than a few mm.
- deleted 2y ago[deleted]
- keepamovin 2y agoIt would be great if the electric effects of spinning fast could result in a reduction in inertial mass to counteract the tendency to disintegrate
- keepamovin 2y agoI suppose another idea is what if you used a fluid? - A non-Newtonian fluid - A ferrofluid - A non-Newtonian ferrofluid - Or, a liquid metal, like: - gallium, or - mercury as the spin mass
- feoren 2y agoThen there's even less holding it together and it would just immediately sploosh everywhere, right?
- keepamovin 2y agoMaybe? From one point of view, I guess you're right. But I'd want to test it. Hehehehe :)