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yesterdays-xkcd · 9 months

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You spin me right round baby, right round, in a manner depriving me of an inertial reference frame. Baby.

Centrifugal Force [Explained]

Transcript Under the Cut

[James Bond, drawn as Cueball, is strapped to a giant wheel suspended from the ceiling. Black Hat is standing next to two levers.] Black hat: How do you like my centrifuge, Mister Bond? When I throw this lever, you will feel centrifugal force crush every bone in your body.

[Same scene, but a closer shot.] Bond: You mean centripetal force. There's no such thing as centrifugal force. Black hat: A laughable claim, Mister Bond, perpetuated by overzealous teachers of science. Simply construct Newton's laws in a rotating system and you will see a centrifugal force term appear as plain as day.

[Closer shot, only Bond's head is visible.] Bond: Come now, do you really expect me to do coordinate substitution in my head while strapped to a centrifuge? Black hat: No, Mister Bond. I expect you to die.

#xkcd #xkcd 123 #centrifugal force #2000s #2006

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abalidoth · 2 years

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Efficiency

So there's a way to interpret gas mileage. In the United States, we usually measure gas mileage in miles per gallon (mpg) which is a measure of efficiency, not usage. The higher the number, the less gas the car uses. So, a proper usage measurement would be gallons per mile. The weird thing about that, though -- Gallons are a unit of volume, and miles are a unit of distance, so the gallon per mile is actually a unit of area. One gallon per mile is about 2 square millimeters, so a 20 mpg car (similar to my gas car) has a gas mileage of about 0.1 mm^2.

This seems like just a weird numerical trick, but this does actually have a meaning -- if your car was leaving a "thread" of gas behind it instead of burning it, the gas mileage is the thickness of the thread. This is explored in this excellent XKCD what-if.

This is something I've thought about a lot over the years, but something happened recently that made the whole thing more complicated.

Emma and I got an electric car.

Now, the efficiency of an electric car is typically measured in the bizarrely mixed unit of "miles per kilowatt-hour". The watt is a measure of power (energy/time) equal to one joule per second. So the kilowatt-hour is exactly (3600 sec/hr)*1000 = 3.6 million joules. The "mile per kilowatt-hour" is kind of a horrific chimera of four different units, some of which are metric and some of which are imperial. It's a weird one.

But, similarly to the gas car, this is a measure of efficiency, and so inverting it gives us our unit of usage. That would be kilowatt-hours per mile. But, similar to how the gallon as a unit of volume is (length^3), the kilowatt-hour as a unit of energy is (mass*length^2/time^2). For example, the joule, the SI unit of energy, is exactly 1 (kg m^2/s^2). But this means that energy divided by length is (mass*length/time^2) -- that's a unit of force! The kilowatt-hour per mile is a unit of force equal to about 2200 newtons. Our electric vehicle, which gets about 4 miles to the kilowatt hour (or 0.25 kWh/mi) has an "energy usage force" of 550 newtons, or 123 pound-force (that is, the force of gravity on a 123 pound object.)

Kinda like the gas-thread interpretation of gas mileage, this interpretation of energy usage can actually be visualized. This is kind of an "aggregated friction force" representing all of the various sources of inefficiency in the car, the force it has to overcome in order to move forward. 123 pounds is shockingly small to me for the force acting against an entire car -- that's the weight of a fairly small person. I could theoretically lift that much, although not easily.

Now, for the finale, I realized something -- we have a force for the usage of an electric car, and an area for the usage of a gas car. Which means, when you're considering whether to change from using a fossil fuel car to an electric vehicle, you want to compare the efficiency of the two. And when you want to compare things, you use a ratio.

The ratio of a force and an area is a pressure.

The ratio between our EV's energy usage and our other car's gas usage is 5.5 billion pascals of pressure, or about 55,000 atmospheres of pressure.

5.5 gigapascals is a lot -- it's about five times more than the highest pressure chemical reactors, or about one third of the pressure that artificial diamond presses use. The physical interpretation is less clear here, though. I guess if you compress the gas car's "gas tube" to 5.5 GPa, it would exert the electric car's "friction force"? Also, units of pressure are used for tensile strength -- 5.5 gigapascals is very close to the tensile strength of the high-performance superpolymer Zylon -- but I don't have a great sense of what that means.

I don't really have a satisfying ending to this, other than that the next time someone asks me why I got an electric car, I can tell them that it's FIVE GIGAPASCALS better than an internal combustion engine!

#units are fun #there's no point to this but if there was a point it would be driven by FIVE GIGAPASCALS OF PRESSURE #math stuff #tesserants

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mostlysignssomeportents · 7 years

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XKCD's What If: "Dear Abby for Mad Scientists" in book form #3yrsago

The book-length version of Randall "XKCD" Munroe's brilliant What-If? column -- which features scientifically rigorous, utterly absurd answers to ridiculous hypotheticals -- has been on the bestseller lists since it was announced in March. Today, it hits shelves and: It. Is. A. <blink>Triumph</blink>.

If you read the column, you know the drill: strangers on the Internet ask Munroe weird, extreme hypothetical questions, like what would happen if you could throw a baseball at the speed of light, or what it would take to make an astronomically plausible asteroid of the size and density of the one that The Little Prince lives on. Then Munroe -- a former NASA roboticist who trained as a physicist -- answers these questions with a kind of fiendish comprehensiveness, describing the bizarre physics of very small, very large, very fast, very slow, very empty and very dense things in meticulous detail that does nothing to mask his near-satanic glee in the perversity of the universe at its extremes.

There's no learning like the learning you do when you're laughing. And if the laughter is inspired laughter, laughter at the vastness and strangeness and sheer delight of the universe, then the learning sticks, because it is bonded to an intense emotion. Reading Munroe doing science is that kind of intense experience, that kind of learning. Every one of these short, lucid, illustrated science articles is a lesson about physics that's not only memorable, it's a pleasurable memory, tinged with tingly, delicious terror-at-a-distance for the howling, eschatological dementedness of physics.

The book gathers together dozens of the best articles from the site, and adds in several more that are new to the collection, as well as a series of highly amusing interludes that present the weirdest questions Munroe has received:

Converting a webcomic -- especially one that is as radical in format as XKCD, which won a Hugo award this year for an animated science fiction story that loaded about one frame per hour for 123 days -- to print format is a tricky business. XKCD Volume 0, Munroe's first print collection, did a lot of clever tricks to ease the transition. In What If, there are fewer puzzles (that I noticed), but the production values of the book are fabulous.

What If features a wraparound, double-sided jacket (the inside of it is a map of the Earth's surface as it would appear if all the oceans were drained through a transdimensional portal and poured onto the surface of Mars); printed boards, and endpapers that are a mosaic of funny, twisted doodles in Munroe's familiar scrawl. This is a book intended to be treasured and to be given as a gift, perhaps one that kicks off a journey into the weirdest, best places in science. There is enough material here for a hundred classroom workshops and a thousand cubicle doors (I have seen Munroe's cartoons on the doors of every scientific institution I've visited, from CERN to the Human Genome Project).

There is, incredibly enough, an audiobook of What If?, which is a weird idea, given how much the explanations rely on Munroe's charming diagrams. But the book is read by Wil Wheaton, who is, for my money, the best audiobook narrator working today, and it was produced by Blackstone audio and recorded at Skyboat in Los Angeles, who do outstanding work, and they all labored mightily with Munroe to turn the diagrams into spoken word (and there's an accompanying PDF, which also helps).

It's easy to inspire a sense of wonder about the universe, but Munroe's gift for inspiring a sense of delight is a rare treasure indeed.

What If?: Serious Scientific Answers to Absurd Hypothetical Questions [Book]

DRM-free audiobook [Read by Wil Wheaton]

https://boingboing.net/2014/09/02/xkcds-what-if-dear-abby-f.html

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psychic-readingonline · 5 years

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Psychic Killer Meaning

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