Category talk:Dimensionless numbers

message: I have removed Avogadro's number as it is not dimensionless.  It is atoms per gram molecular weight or per kilogram molecular weight, so its value depends on the size of the gram in one case, or the kilogram  in the other.  If we used pounds or ounces, again Avogadro's number would change in value.  For example, in the case of ounces, it would be the number of atoms in a number of ounces equal to the atomic weight (which, itself is dimensionless, but the ounce is not)
Pdn 05:33, 9 Mar 2005 (UTC)
thread_title:  Avogadro's number 

message: Isn't this a completely unnecessary page? I can come up with tons of dimensionless numbers. Euler characteristic, speed of light and whatnot. (anonymous)
Of course, pi is a dimensionless number, and so is Catalan's constant. Note that the nathematical examples are constants. But in physics and engineering, and related sciences such as astronomy and chemistry, dimensionless constants are particularly useful for describing and classifying physical behaviors.  Such numbers (generally not constant from case to case) can tell you whether a fluid flow is viscous or inviscid (i.e. negligibly viscous), whether an airplane is travelling subsonic or supersonic (Mach number > 1), and a chemical example is on that page, too.  Unfortunately, the majority of listed numbers are from fluid mechanics and other important cases are omitted.  For example, for a rotating deformable body such as the Earth or Sun, there is a dimensionless number J2 that gives you a measure of the deformation (equatorial bulge - but for Saturn, small for the Sun). I don't have time to put in more examples like that one, but there are lots of useful ones (power factor in electric transmission theory is one) and with that page as a sort of stub, people can add more. It is very frustrating to students to run into a Peclet or Prandtl number and not find its definition easily; on the other hand it can be called a "cottage industry" to collect these and add more.
Pdn 03:44, 15 Apr 2005 (UTC)
thread_title:  is this category useful? 

message: Message originally posted on User talk:SebastianHelm
Dear Mr. Helm,
Glad that attention is going to those pages.  I think you should restore engineering and science as categories. In engineering, the usage of dimensionless numbers reduces large families of problems that do not seem identical into single cases.  It's an approach that many engineers have trouble dealing with; they will produce charts with a half dozen parallel lines that could be collapsed to one by scaling the axes. So it is educational to reference the page from engineering.  In science the usage is just as important and more frequent. Unfortunately, there's an apparent (but largely fictitious) difference between pure maths constants and physical dimensionless numbers. For example, pi=3.1415926535... seems to be a mathematical constant with no dimensions, but it can be regarded as having the dimensions of inverse radians. To get a perimeter from a radius you have to measure the included angle in radians and use that value of pi I gave.  If you measure the angle in degrees, you must use a constant numerically equal to pi/180, not the number just quoted. (I know this is arguable as you can define pi as the ratio of the circumference to the diameter, but for practical use it comes out as I said.) Anyway, I don't see where it hurts to leave the old categories and just add new ones, too.
Pdn 14:57, 20 May 2005 (UTC)
thread_title:  categories for dimensionless numbers 

message: I am not really familiar with the structure of categories and subcategories in Wikipedia, so do whatever you think is best. I do not see how these numbers are related to "measurement" but evidently you do.  Dimensionless numbers are very important in physics (e.g. the fine structure constant, the ratio of gravity strength to electric field strength, mass ratios of fundamental particles are essential).  Being important in gas dynamics (Reynolds, Mach and similar numbers) they are important in astronomy, because most of the material in the universe is in gaseous form, and often it is stirred or shocked.  I do not know much biology but I suspect there are important dimensionless numbers there.  I will ask a chemist friend about chemistry - see you later Pdn 01:12, 21 May 2005 (UTC)
thread_title: reply

message: I added Pi.  About this one, I sure hope we don't go round and round and round and r...  Bob Stein - VisiBone 22:48, 1 August 2007 (UTC)
thread_title: Pi

message: Shouldn't Euler's number e (approximately 2.71828) be here?  Apparently in addition to the Euler number (physics) aka Cavitation Number.  Not to be confused (!) with Euler numbers (integer series) nor Eulerian numbers (a triangular table of integers). Bob Stein - VisiBone 22:48, 1 August 2007 (UTC)
thread_title: e?

message: In Dimensionless quantity, why is the modal "may" used? Either it is or isn't. --Backinstadiums (talk) 12:57, 19 August 2019 (UTC)
thread_title:  ...may also...