https://www.explainxkcd.com/wiki/api.php?action=feedcontributions&user=162.158.222.16&feedformat=atomexplain xkcd - User contributions [en]2020-11-28T11:06:48ZUser contributionsMediaWiki 1.30.0https://www.explainxkcd.com/wiki/index.php?title=2073:_Kilogram&diff=1660662073: Kilogram2018-11-16T23:19:02Z<p>162.158.222.16: equation not resulting in usable value (explained)</p>
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<div>{{comic<br />
| number = 2073<br />
| date = November 16, 2018<br />
| title = Kilogram<br />
| image = kilogram.png<br />
| titletext = I'm glad to hear they're finally redefining the meter to be exactly three feet.<br />
}}<br />
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==Explanation==<br />
{{incomplete|Created by a CONSTANT PLANCK. Links to resources would be good. Please mention here why else this explanation isn't complete. Do NOT delete this tag too soon.}}<br />
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On the day of this comic, the {{w|International Committee for Weights and Measures|International Committee for Weights and Measures}} voted to redefine the kilogram by fixing it to the value of Planck's Constant. This is done by passing a measured current through an electromagnet to exert a force to balance 1 kg. The change will take effect on May 20, 2019, when the platinum cylinder International Prototype Kilogram that defines the unit will be retired. This means that the mass of a kilogram will no longer be tied to a physical object, but to the fundamental properties of the universe.<br />
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The previous method of confirming that a kilogram is accurate is to use physical metal weights measuring exactly one kilogram, periodically transporting them around the world to an official weight lab to confirm they still weigh the same. Over time these physical objects have changed very slightly in their mass making them unreliable in the long run -- thus running into the issue that a kilogram did not stay a constant measure of mass. Note that these weights and comparisons are so precise that a fingerprint on one of the weights could throw them off.<br />
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In this comic, Black Hat announces that the kilogram has been redefined as equal to one pound. Ponytail and Cueball seem to think this makes things simpler, but Megan is rightfully alarmed. The metric system of measurement is the one used by most of the world and is the standard system used in science. It is considered superior to the {{w|United States Customary System}} and the {{w|Imperial system}} (both of which the pound is part of). Therefore, redefining the kilogram to be based on the pound would make things much, much worse and outrage supporters of the metric system. More to the point, the pound is still often defined by metal weights, thus running right back into the very same problem they tried to escape from.<br />
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In real life, the pound is officially defined as 0.45359237 kilograms, or less than half a kilogram. This makes defining a kilogram as one pound even more impossible as they are then stuck in a loop, as the pound must weigh less than half of a kilogram, but then the kilogram will have to be updated to match the new pound mass. (Although one could argue this is not true: 0.45359237 x = x has a solution, namely 0. To be able to measure using this unit it has to have a non-zero value.).<br />
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The title text continues the joke by saying that the meter has been defined as exactly three feet. The yard, the closest US measurement to the meter, is three feet. However, a meter is about 9 centimeters longer than a yard. As with the pound, the metric system is used to define the yard as it is officially defined as 0.9144 meters.<br />
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==Transcript==<br />
:[Black Hat talking to Ponytail, Cueball, and Megan while all stand in a row. Megan's hands are raised emphatically.]<br />
:Black Hat: To end many years of confusion, the International Committee for Weights and Measures has just voted to redefine the kilogram.<br />
:Black Hat: As of next May, it will equal exactly one pound.<br />
:Ponytail: Oh, cool.<br />
:Cueball: That ''does'' make things simpler.<br />
:Megan: '''''No!!'''''<br />
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==Trivia==<br />
To further expand on this, the classic definitions of all our various units of time, length, mass, and temperature are based on phenomena that are neither convenient to measure precisely nor in fact consistently reproducible. The duration of an Earth day and year vary unpredictably, the circumference of the Earth varies, the International Prototype Kilogram gains or loses mass any time it is handled (and in fact just sitting there it and its reference copies diverge from each other), and the value of baseline temperatures such as the freezing point of water depend on which isotopes of hydrogen are in the water molecules.<br />
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Nevertheless, there really are constants of nature. For example, one of them is ‘''c''’, the speed of light in a vacuum. The expressed value of ''c'' depends on your choice of the unit of distance and the unit of time, but it’s a constant in those units. Now just suppose we all had a reproducible way to define a specific unit of time, which just for fun we call a ‘second’. You might not know the length of a ‘meter’, but if I told you that measured in meters per second the universal constant value of ''c'' is exactly 299792458 meters per second, then I would have fixed the length of a meter to be exactly the distance light travels in a vacuum in 1/299792458 seconds. And in fact this is what the international body responsible for defining our SI units has done.<br />
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One second is defined to be a specific number of certain state transitions of a cesium 133 atom. The specific number was set in the year 1965, so as to match a previous astronomical standard called Ephemeris Time to the limit of human measuring ability at the time. The 1965 definition didn’t change the actual duration of a second, but it did make its measurement forever reproducible.<br />
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In 1983 the value of ''c'' was fixed to the value noted above. Prior to that it had been measured with respect to existing definitions of a meter, and had to be expressed with a measure of uncertainty. For example in 1973 a team at the US National Bureau of Standards refined ''c'' to 299,792,457.4 m/s ± 1 m/s. But from 1983 onwards, with an exact integer value for ''c'' that is quite close to that Bureau measurement, the length of a meter is now fixed with no plus/minus uncertainty. Furthermore, both the second and the meter match their predecessor definitions for all intents and purposes.<br />
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Similar redefinitions of units of mass and of temperature in terms of universal constants have been agreed to, mass with regard to the Planck constant ''h'', and temperature with regard to the Boltzmann constant ''k''. The constants ''h'' and ''k'' had previously been measured quantities, complete with uncertainties. The SI body fixed both of them to exact values, resulting in exact, no-uncertainty values for a kilogram of mass and a kelvin of thermodynamic temperature. As with the second and the meter, these new definitions match their predecessor definitions for all intents and purposes.<br />
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To expand on this even further, three additional universal constants that were previously measured and that had uncertainty values have been assigned fixed values, resulting in exact definitions of three corresponding units of measurement without affecting their applicability. Fixing the unit of elementary charge, ''e'', serves to define the unit of electric current, the Ampere. Fixing the unit of luminous efficacy ''K<sub>cd</sub>'' serves to define the unit of luminous intensity, the candela. And fixing the Avogadro constant ''N<sub>A</sub>'' serves to define the unit of amount of substance, the mole.<br />
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A very recent Wikipedia article about redefining the SI units of measure in terms of newly fixed values of things taken to be universal constants is {{w|Redefinition of SI base units}}.<br />
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{{comic discussion}}<br />
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[[Category:Comics featuring Cueball]]<br />
[[Category:Comics featuring Megan]]<br />
[[Category:Comics featuring Black Hat]]<br />
[[Category:Comics featuring Ponytail]]<br />
[[Category:Physics]]</div>162.158.222.16https://www.explainxkcd.com/wiki/index.php?title=1924:_Solar_Panels&diff=1486951924: Solar Panels2017-12-04T17:39:45Z<p>162.158.222.16: Added reference to solar frigging roadways.</p>
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<div>{{comic<br />
| number = 1924<br />
| date = December 4, 2017<br />
| title = Solar Panels<br />
| image = solar_panels.png<br />
| titletext = This works for a surprising range of sunlit things, including rooftops (sure), highway surfaces (probably not), sailboats (maybe), and jets, cars, and wild deer (haha good luck).<br />
}}<br />
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==Explanation==<br />
{{incomplete|Created by a SOLAR PANEL - Please change this comment when editing this page. Do NOT delete this tag too soon.}}<br />
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This handy decision tree aims to help in finding out whether a given object should have {{w|solar panel}}s installed on it.<br />
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The root question is whether the object of choice moves. If it doesn't and has no nearby empty space where there would be more practical for the solar panel installation, then yes, the object should be equipped with the solar panels. If the object is static, but you could install the panels somewhere else nearby, probably that's the best place.<br />
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If the object moves, the next question is whether its batteries can be recharged or swapped with ease, in which case batteries may be a better option than solar panels.<br />
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Finally, if the object moves and batteries are not an option, the last question is whether the object heats up during operation. If not, solar panels may be an option. If it gets hot, [[Randall]] doubts it mockingly.<br />
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The title text suggests that this flow chart is very broadly applicable to anything the Sun hits; however, the flow chart does not mention if the thing in question actually ''needs'' solar panels.<br />
"Highway surfaces" are likely a reference to the failed project "Solar Roadways". A project known for high ambition, a very successful crowdfunding campaign, and hash (but correct) criticizes on its feasibility.<br />
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==Transcript==<br />
{{incomplete transcript|What about screen readers? Do NOT delete this tag too soon.}}<br />
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<big>Should I put solar panels on it?</big><br />
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Does it move around?<br />
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yes no<br />
| |<br />
Does it have Is there an empty space nearby<br />
regular chances where it would be easier to<br />
to recharge or put them?<br />
swap batteries? | |<br />
| | | |<br />
no yes yes no<br />
| | | |<br />
When running, is it Probably Sure<br />
hot to the touch? not <br />
| |<br />
no yes<br />
| |<br />
Maybe Haha<br />
good luck<br />
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{{comic discussion}}<br />
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[[Category:Flowcharts]]</div>162.158.222.16https://www.explainxkcd.com/wiki/index.php?title=Talk:1831:_Here_to_Help&diff=139467Talk:1831: Here to Help2017-05-02T00:31:26Z<p>162.158.222.16: </p>
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<div><!--Please sign your posts with ~~~~--><br />
So who else read the "Six months later" caption in the voice of the French narrator from SpongeBob Squarepants? [[Special:Contributions/172.68.58.41|172.68.58.41]] 23:26, 1 May 2017 (UTC)<br />
: So I'm not the only one who does that! [[User:Dontknow|Dontknow]] ([[User talk:Dontknow|talk]]) 00:00, 2 May 2017 (UTC)<br />
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Gosh, is Randall making a parallel to someone else who only recently announced that his job is hard, and that nobody knew how complicated things could be? Seems like a clear poke at Trump to me. [[Special:Contributions/108.162.246.23|108.162.246.23]] 23:43, 1 May 2017 (UTC)<br />
:EVERYONE feels like that after the election. Get over it. [[User:Jacky720|That's right, Jacky720 just signed this]] ([[User talk:Jacky720|talk]] | [[Special:Contributions/Jacky720|contribs]]) 23:50, 1 May 2017 (UTC)<br />
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Between algorithms and "objectively" establishing that a problem is hard, I took this to be a reference to https://en.wikipedia.org/wiki/NP-hardness … --[[Special:Contributions/162.158.222.16|162.158.222.16]] 00:31, 2 May 2017 (UTC)</div>162.158.222.16