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People generally do a shit job maintaining their car where the worst case scenario is generally that it just doesn't start. Having a fifteen pound weight spinn
by selectodude 4y ago
People generally do a shit job maintaining their car where the worst case scenario is generally that it just doesn't start.
Having a fifteen pound weight spinning at 30,000 RPM being maintained as well as your average vehicle sounds utterly terrifying.
- NegativeLatency 4y agoImagine what would happen in a crash Obviously there’s energy from batteries or gasoline or whatever in regular cars
- SketchySeaBeast 4y agoBut this would equip every car with a detached semi tire (or possible more)'s worth of kinetic energy to go flying.
- deleted 4y ago[deleted]
- frankus 4y agoIndeed. Kinetic energy in the form of a flywheel has both the advantage and the drawback that it’s relatively easy to release all of the energy in a fraction of a second. Of course you can do the same with chemical energy, but we have good intuition around things that will detonate. With everyday substances, both a stick of butter and an SUV traveling at 100 mph contain about a kilowatt-hour of energy, but the latter would generally be more destructive if something goes wrong.
- nerdponx 4y agoIf anything, it's astounding just how much energy is in food like butter, and in such a stable configuration. Alternatively, it's humbling how bad and inefficient we are at manipulating the physical materials in our environment, because we can't so much as replicate a stick of butter without making it noisy and dangerous.
- zmgsabst 4y agoWhat do you mean? A butter stick sized piece of C4 is similarly stable but contains more stored energy.
- rdl 4y agoI thought it was "via a cow", which is noisy and dangerous.
- dtgriscom 4y agoDoes the C4 have more energy, or just the ability to release it (a whole lot) more rapidly?
- HarryHirsch 4y agoThe latter. High explosives come with the oxidant built-in, and the reaction propagates faster than the speed of sound.
- causi 4y agoYou are completely wrong. 1 gram of C4 contains 1.59 kilocalories of energy. 1 gram of butter contains 7 kilocalories of energy.
- saalweachter 4y agoIt looks like C4 is 60% denser than butter, but that isn't enough to save the original claim on size, versus weight.
- dredmorbius 4y agoExplosives, much as other forms of energy storage, tend to have lower specific energy (energy per unit mass) than slower-burning fuels. 1 kg of TNT has roughly 4.6 MG. 1 kg of petroleum has roughly 45 MG, ten times as much. The difference is that the trinitrotoluene combusts rather more rapidly than the oil. But if, say, you simply want thermal energy and have a mass budget, you'll go with the oil. C4 has an energy density of about 5.1 MG/kg, slightly greater than TNT, but only 1/8th that of petroleum. <https://en.wikipedia.org/wiki/C-4_(explosive) https://en.wikipedia.org/wiki/C-4_(explosive)> <https://en.wikipedia.org/wiki/Energy_density https://en.wikipedia.org/wiki/Energy_density>
- chrisweekly 4y ago"both a stick of butter and an SUV traveling at 100 mph contain about a kilowatt-hour of energy" How's that?
- Cerium 4y agoThe stick of butter is easy: it contains about 800 kilo-calories. This can be converted to 0.94 kWh: (calories)(joules per calorie)(watts = joules/second)(kw = 1000w) Kinetic energy is 1/2(v*v^2) The average SUV weighs about 2000 Kg. That yields about 0.55 KWh. Perhaps the parent used a larger SUV.
- chrisweekly 4y agoIt still seems bonkers to me. What if the SUV slowed to a stop, standimg at rest beside the stick of butter. Which represents more potential energy?
- Cerium 4y agoBesides the gas in the tank the car has zero potential energy at a stop. All the kinetic energy was lost to heating the brakes.
- frankus 4y agoThe parent forgot the 1/2. So more like 140 mph (or a heavier vehicle).
- TylerE 4y ago15 pounds? Not gonna store much energy. Many cars already have several pound turbo impellers spinning at 50k+ rpm. (And remember force will be proportional to rpm^2)
- bagels 4y agoI don't think there are many turbo impellers that heavy, are there? The ones I've disassembled we're not very heavy from what I can recall.
- dtgriscom 4y agoThe energy also depends on the diameter; presumably the flywheel would be larger than a turbo rotor.
- dredmorbius 4y agoTurbos are small and lightweight. The goal is for them to respond quickly to changes in requested power, with minimal lag (already a problem). So small and light, with effective (air) pumping power. If your turboimpeller is doubling as kinetic energy storage, you're doing it wrong.
- winrid 4y agoI think parent comment's point is that they already have a dangerous spinning object. Not that I think this is a good counter argument...
- mrguyorama 4y agoIt's not accurate. Turbo impellers should not be heavy enough to store significant energy, except maybe in large diesel engines, and are also naturally housed in a large, often cast iron, housing that will easily contain any shrapnel.
- dredmorbius 4y agoTurbos also run periodically and reasonably infrequently, at least for street vehicles. They're invoked generally under high-acceleration or load, not when cruising at constant speed. A flywheel would typically be spinning not only far more often but at great speed and load.
- userbinator 4y agoA bearing failure is probably the most common failure mode, would probably result in an unignorable insanely loud noise, and render the unit inoperable once they actually seize.
- WalterBright 4y agoI wouldn't bet on it. Never stand radially from a high performance engine.
- WalterBright 4y agoI replaced the cast aluminum bell housing on my dodge with a forged steel one, probably about 9x stronger. I've seen pictures of flywheels sawing through the body of the car. I like my feet.
- HarryHirsch 4y agoAll biologists are shown photos of exploded ultracentrifuges. It makes them care for centrifuge rotors, the consequences when limits are exceeded are spectacular.
- dncornholio 4y agoThey are already inside almost all cars though. This is just one with magnets.
- lathiat 4y agoRelevant video from Veritasium today on kinetic energy: https://youtu.be/J_n1FZaKzF8 https://youtu.be/J_n1FZaKzF8 Also this is a problem enough at <10,000 RPM with just normal metal flywheels and clutches that many drag racing and Motorsport rules require extra “scatter shields” to protect both drivers and spectators from them exploding or exiting from the normal bell housing.