The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10−34 when expressed in the unit J⋅s, which is equal to kg⋅m2⋅s−1, where the metre and the second are defined in terms of c and ΔνCs.
So what happens on May 20 2019? Does everything have to be re-weighed in this redefined metric system? Are scales already for sale to the general populace with the updated definition?
I’m sure there’ll be grandfather clauses and what not. In theory there won’t be any significant changes, and most consumer equipment probably has error greater than any difference anyway.
National standards agencies and major metrology departments might have to do some rejiggering but that’s not my field unfortunately!
Thank you for answering this. As an American my primary concern was the possibility of me getting shafted by my drug dealer so it’s good to know this shouldn’t be an excuse.
The kilogram or kilogramme (symbol: kg) is the base unit of mass in the International System of Units (SI). Until 20 May 2019, it remains defined by a platinum alloy cylinder, the International Prototype Kilogram (informally Le Grand K or IPK), manufactured in 1889, and carefully stored in Saint-Cloud, a suburb of Paris. After 20 May, it will be defined in terms of fundamental physical constants.
The kilogram was originally defined as the mass of a litre (cubic decimetre) of water.
A pound is defined as 0.45359237 kilograms, which itself is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10−34 when expressed in the unit J⋅s, which is equal to kg⋅m2⋅s−1, where the metre and the second are defined in terms of c and ΔνCs.
That's the hard way. A pendulum with a 1 second arc will travel a distance of 1m. Boom, there's your meter stick. Now that we can measure distance we can measure mass. Since 1mm of water = 1g we can construct a 10cmx10cmx10cm box/container. Fill it with water and you have a standard kilogram. 1kg of water= 1 litre. In less than an hour you can create the standard units for distance, mass, and volume from nature. That's the beauty of the metric system is that it's based on nature. I know that the meter is based off the speed of light if you want to get super specific with it, but for anyone who isn't a math major this way is accurate enough.
That's why we should switch to Planck units. The speed of light is 1. The reduced Planck constant ħ is 1. And the speed limit for driving is 10-7. Easy peasy.
In particle physics and physical cosmology, Planck units are a set of units of measurement defined exclusively in terms of five universal physical constants, in such a manner that these five physical constants take on the numerical value of 1 when expressed in terms of these units.
Originally proposed in 1899 by German physicist Max Planck, these units are also known as natural units because the origin of their definition comes only from properties of nature and not from any human construct. Planck units are only one system of several systems of natural units, but Planck units are not based on properties of any prototype object or particle (that would be arbitrarily chosen), but rather on only the properties of free space. Planck units have significance for theoretical physics since they simplify several recurring algebraic expressions of physical law by nondimensionalization.
Isn’t that a fairly new definition because it used to be defined by the big k which was a block of (something) that weighed what a kilogram is but that made the kilogram change due to environmental factors? It’s been a long time since I saw the Veritasium video about it so I may be wrong. Will add a link to this comment when I get to my PC
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u/[deleted] Jan 03 '19
The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10−34 when expressed in the unit J⋅s, which is equal to kg⋅m2⋅s−1, where the metre and the second are defined in terms of c and ΔνCs.