Units and Constants 

General Remarks 

This is the nononsense module with the hard facts about units, constants and transformations from one system of units into an another one.  
No explanations, historical roots, really outdated or unusual units are given  for the fun part use this link.  
First, some basics about measurements and units.  
In physics we always have two things: a physical quantity  e.g. the speed of something, or the strain of something under load  and some units to measure the quantity in question.  
The physical quantity is what it is. It does not depend on how you express it in numbers. Somebody on some other planet will do it differently from you and me for sure  
The number you will give to the physical quantity is strictly a function of the units that you chose. You might use m/s, lightyears/s or wersts/year  that will just change the number you assign to the speed of the moving object but not the speed itself. Trivial, but often forgotten.  
To make life easier for everybody (at least for scientists), the choice of units was taken away from you and me. Everybody is now required to strictly adhere to the international standard system, abbreviated in any language as SI units.  
Well, by now I and most others scientists, do comply with the SI system (which was not always the case); about you I don't know. The public at large, of course, does not give shit; especially in the USA. Tell the gas station attendant any number you like for the tire pressure in Pascal, and he (or she) will just look at you as if you escaped from the lunatic asylum. Its psi or bust! On occasion, even (american) engineers or scientists do not use SI units  with disastrous consequences like satellites lost in space.  
The question now is: how many basic units do we need, so we can express everything else in these units? And which ones do we take?  
This is one of the deeper questions of humankind. Physicists claim that we just need one more truly basic fundamental constant of nature  and we do not need units at all anymore. Velocities, for instance, can always be given using the absolutely constant speed of light (in vacuum) as the unit with the numerical value 1. Your typical car speed than would be something like 0,000001.  
But redundancy tends to make life easier and more pleasant for some (just look at your typical Sheik and his harem). The SI system gives us 7 basic units which are independent of each other and plenty of derived (and thus redundant) ones.  
Basic Units 

Here are the seven basic units of the SI system:  


From this basic units all other SI units can be derived. Below are tables with the more important secondary units.  
First, we look at some secondary units just invoking basic units and a length. While we often do use special symbols for these quantities (e.g. r for density), these symbols are not really necessary and thus were not pronounced immutable and sacred as, e.g., the "m" for meter or the "s" for the second.  


Now let's look at more involved units  including important quantities like energy, voltage, and magnetic things.  
They are more involved, because we usually do not express them in SI basic units  which is perfectly possible  but in secondary units. We will also find one case where there is no unit  it just cancels out.  
These units often have their own symbols for reasons that become clear if you look at the SI units. These symbols should not be used for something else  

Mercifully, the members of the "Comite International des Poids and Mesures" are human (up to a point). They did not outlaw all older units in one fell stroke, but sorted them into three groups:  
"Old" units which may be used together with SI units without restrictions.  
Old units which may be used for some time in parallel to SI units.  
Old units which are definitely out and must not be used at all any more.  
Some of the units in the second category are regional and you probably have never heard of them. I will not include them here. The number of outlawed units is legion, we just include the still tempting ones.  
Here is the first category: Some of the nonSI units you still may use without restrictions:  


Now to the old units you may use for some more time to come in parallel to the SI units:  


Now to the units you must not use anymore!. We might put them into two groups:  
1. The forerunners of the SI units, the cgs units; i.e. the units based on the centimeter, the gram and the second.  
2. The simple old fashioned nono's.  
While it may appear that the cgs system is practically the same as the SI system, this is not so!  
Of course, the cm, g, and s are essentially the same basic units as in the SI system, the abbreviation "cgs", however, does not tell you anything about the other necessary basic units in this system  and that is where the problems come in!  
In fact, there were several cgs systems  the electrostatic, the electromagnetic, and the Gauss cgs system! I will not go deeper into this, however.  
Finally, some still fondly remembered old units you simply do not use anymore:  


Fundamental Constants 

Fundamental constants are some numbers with units that cannot (yet) be calculated from some physical theory but must be measured.  
This may have three possible reasons:


Hotshot physicists have some ideas, which constants might fall into which category. Speculations along this line are a lot of fun  but of no consequence so far. So I will not dwell on this. Of course, you may check for yourself which one of the three possibilities you are going to embrace and thus get some idea of what kind of person you are.  
Fundamental physical theories usually introduce one new fundamental constant. Mechanics (including gravitation) needs the gravity constant G, quantum theory has Plancks constant h, statistical thermodynamics introduces Boltzmanns constant k, the special theory of relativity (or Maxwells theory of electromagnetism which is really part of the relativity theory) needs the speed of light c.  
New theories sometimes "explain" old constants of nature because they can calculate them, or replace them by something more fundamental. Boltzmann's constant k, for example, is more fundamental than the "fundamental" gas constant R, because it relates its number to a fundamental unit of matter (1 particle) and not to an arbitrary one like 1 Mol.  
How many truly fundamental constants are there? Why do they have the values they have? Interestingly, just slight deviations in the values of some constants would make carbon based life impossible; this is where the socalled "anthropic principle" comes in. Will we eventually be able, with a "Theory of Everything" (TOE) to calculate all natural constants?  
Nobody knows. We run against the deepest physical questions at this point.  
So let's just look at what we have. Since it is customary to list as natural constants also some quantities that are actually computable from others by now, I include some of these "constants" here, too (together with the conversion formula).  

3.1.4 A Bit More About Tensile Testing
Group 2 / IIA; Alkaline Earth Metals Group
Group 12 / IIB; Scandium Group
Group 12 / IIB; Titanium Group
Group 7 / VIIB; Manganese Group
Group 8  10 / VIIIB; Iron  Platinum Group
Group 16 / VIA; Chalkogenides or Oxygen Group
Group 3 / IIIB; Lanthanides or "Rare Earths"
Spring Model and Properties of Crystals
Ductile to Brittle Transition or Cold Shortness
3.1.1 Breaking Things in Style
Smelting Science  5. Smelting Details 2
© H. Föll (Iron, Steel and Swords script)