5 ms·
Indeed. 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
by frankus 4y ago
Indeed. 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>
- HPsquared 4y agoThis is because for the butter, you're not counting the weight of all the oxygen that is needed for combustion. 1kg of butter requires several kg of oxygen (or if you're using air, even more weight if you count all the inert nitrogen)
- dredmorbius 4y agoThat's a valid point and does shift the balance somewhat, but not fully in favour of nitrogen-based explosives. From a practical perspective, at least within Earth's atmosphere, that oxygen is "free". There's not need to carry it with you, though some combustion applications may require feeding it at high rates (turbochargers, compressors, gas turbines, and the like). Most automobiles functions strictly under natural aspiration, and even industrial combustion typically utilises relatively low-power bellows or air introduction. Rockets, of course, carry their own oxidizer, so we can compare energy densities based on net yield. The Saturn S-IC (first stage of the Saturn V stack) is powered by the combustion of RP-1 (a highly-refined grade of kerosene) and LOX, liquid oxygen. The tank sizes were 730,000 L fuel, for which I'm assuming an average density of 0.9g/mL (0.81--1.02 given by Wikipedia), and 1,204,000 L oxidizer, at 1.41 g/mL. That works out to 657 tonnes of RP-1, 1,374 tonnes of LOX, or a total of 1,511 tonnes. The fuel alone is 43.5% of the total mass, which means total specific energy is reduced to 17.63 MJ/L, which is still about 3.5--4 times greater than TNT or C4, by comparison. There's one further question as to whether or not the rocket is tuned to run rich, stoichometric (balanced fuel and oxidizer), or lean. Most LOX rockets burn somewhat rich, based on recollection and Wikipedia, so the LOX quantity is slightly short of what might support complete combustion, though not enough to affect the overall outcome here. <https://en.wikipedia.org/wiki/S-IC https://en.wikipedia.org/wiki/S-IC> <https://en.wikipedia.org/wiki/RP-1 https://en.wikipedia.org/wiki/RP-1> <https://en.wikipedia.org/wiki/Liquid_oxygen https://en.wikipedia.org/wiki/Liquid_oxygen> <https://en.wikipedia.org/wiki/Rocket_propellant#Mixture_ratio https://en.wikipedia.org/wiki/Rocket_propellant#Mixture_rati...>
- fsckboy 4y agoooooh now I understand, I thought GP meant that both a stick of butter and an suv, both travelling at 100mph, had the same kinetic energy and I was scratching my head as to how big that stick of butter was.
- marmetio 4y agoFamily-sized
- WalterBright 4y agoMy dad told me a story about some British pilots who were pitching in to help the Soviet air force during WW2. The day the temperature dropped below freezing, the women brought out big kettles full of fat to feed the pilots. The RAF pilots were disgusted. But after flying at altitude in the cold, they relished eating the fat. It was loaded with calories that kept them warm. I noticed something similar when I'd go skiing. Loading up with eggs and butter for breakfast meant I'd never feel cold.
- kjellsbells 4y agoProbably no accident that old-school ski cafes on the slopes serve up calorically dense food like cheesy omelettes and fondues... Talking of old-school, I am reminded of Robert Wood's The 2oz Backpacker from 1980-something that advised hikers to take a swig of cooking oil before going to bed, for calorie-burning warmth.
- NegativeLatency 4y agoIdk if I could stomach the cooking oil, but extra virgin olive oil or preferably some salted butter would be more palatable. Cheese sounds even better still but not as calorie dense.
- HPsquared 4y agoReally it's not just the butter though, it's the combination of butter + oxygen - there is quite a lot of oxygen needed. Start putting it in tanks like rocket fuel, and it looks more 'energetic'.
- 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).