# Tag Info

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The mass flow rate $Q_m$ and the cross-sectional area $A$ are almost enough to give you this. The only other thing that you need is the density of the water $\rho$. Since $m=\rho V$, we have that $Q_m=\rho A v$, where $v$ is the flow velocity. To see this, think about the amount of water that passes through a square hole over a time period of one second. ...

-1

I believe that you need to be more specific by what you mean by "power of the river". If you mean "how much power could be gained from the river by putting the whole thing through an electrical turbine" then the important quantities are the mass per second, the altitude above sea level and the Earth's gravitational field constant (little g).(Which give the ...

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Everything!!! Natural units just make your life simpler, is like removing every unnecessary conversion factors which need to be added, helps by making the equation look smaller, and can easily give back the final equation just by dimensional analysis. Okay you might be wondering what dimensional analysis has got to do with natural units. Well let's see an ...

2

Similarly you can ask about why speed of light was defined to be $299792458 ms^{-1}$ and not something else? Simple answer is that doing such would change the unit of length $m$. How? Simply if you make it (say) $1000000000ms^{-1}$ then the new meter would be around $3.33564$ times of the previous meter! This means that if you previously bought $1m$ of ...

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There are many, many instruments which are calibrated using the old definition of the kilogramme - the International Prototype Kilogramme (IPK) made of a platinum-iridium alloy. So one kilogramme measured using the new definition had to be as close as possible to one kilogramme using old definition so as not to have to recalibrate all instruments which ...

9

It is an updated experimental value that matches measurements by Kibble balances and by counting atoms in silicon spheres to determine Avogadro’s number. The 2010 measurement was presumably consistent with only one metrological approach. The new exact value is consistent with both. See this NIST page, which says The Kibble balance and Avogadro project ...

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1 mol is defined as Avogadro's number $6.022\cdot10^{23}$ and you could count anything using moles. We could count some events happening in a fixed time, for example water molecules flowing through a pipe. If we divide the total count of those molecules by elapsed time we get rate of flow which in this case would measured in $\frac{mol}{s}$. If we wanted to ...

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The answer by Michael Deckers provides some useful references, but is not completely correct. The Rapport BIPM-2019/05 linked in that answer does give Spectral luminous efficiency functions in sections 4.1 through 4.3 and tables 1 and 2 — for all of photopic (2° and 10° FOV versions), scotopic and mesopic vision, the latter for varying levels of adaptation....

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In the SI system of units, mole has a dimension. But there is a unit system where the mole is a dimensionless number. See https://www.ncbi.nlm.nih.gov/pmc/articles/PMC61354/.

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In the SI system of units, electric current is a base unit. There is a system of units that uses charge instead of electric current as a base unit. See https://www.ncbi.nlm.nih.gov/pmc/articles/PMC61354/.

2

They aren't "kind of quantities" they are "kinds of quantities". One kind of quantity is lengths. Another is energies. You could substitute "type" for "kind" and say that height and width are two quantities of the same type, instead of quantities of the same kind. The only reason not to is some standards body has chosen to arbitrarily standardize on using ...

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The BIPM booklet on photometry [MEP 2019. p 6] states: "The above definition of the candela applies to photopic, scotopic and mesopic vision." In essence, this means that for any spectral luminosity function whatever, the value for 540 THz is by definition 683 lm/W. In other words, the graphs of these functions, when using SI units, all ...

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