Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] explanatory and response variables (was calibration )




Not to mention abscissa and ordinate :-)

That's a joke in the present context. Students don't know abscissa
and ordinate ... and I know some seeriously famous physicists who
can't keep 'em straight.
-----------------


bc's mnemonic -- ordinate => ordinance, the stuff that blows up. IHRC, was taught ordinate related to gunnery in ROTC (1955). Then when a grad. student (ca. 1964) successfully threatened demonstrations to get rid of mandatory ROTC. Some fools (NOT related to our org.) tried to burn down the building.

-----------------

Example: In the context of magnetic hysteresis, it is OK to say we are plotting B versus H, but strictly speaking B is not a function of H (nor vice versa).
http://www.irm.umn.edu/hg2m/hg2m_b/Image10.gif

-

please explain.


bc, thinks about to be disabused.


p.s. I think ME's description logically conforms to the majority of us. We are right handed and our dictionaries end w/ the order X, Y, and Z.
A machinist friend related his experience w/ an Italian lathe whose cross slide was left handed; disaster.


John Denker wrote:

On 08/08/2007 02:39 PM, Michael Edmiston wrote:


(1) What should we call the axes of a 2-dimensional graph (in general)?

horizontal and vertical
x and y
independent and dependent


Not to mention abscissa and ordinate :-)

That's a joke in the present context. Students don't know abscissa
and ordinate ... and I know some seeriously famous physicists who
can't keep 'em straight.
Along the same lines, more generally one speaks of the /argument/ or
arguments to a function, and the function /value/.


I am amazed how many students don't know horizontal and vertical,


Sigh.


but probably they are more familiar with that than with independent and dependent.


Yes.


x and y are usually understood, and that is usually what spreadsheets call them, but it seems odd to call them x and y when we are not plotting y as a function of x, but perhaps plotting current as a function of voltage.

If we are plotting current as a function of voltage, is it just best to get over my hangup and tell the students that voltage is on the x-axis and current is on the y-axis?


I don't mind the x-axis and y-axis terminology.

You can sell it to yourself as follows: We are making a coordinate
transformation from (current,voltage) space to (x,y) space. This is,
in fact, how graphics software does it. Some sort of transformation
is needed to get the scale factors right, e.g. some number of amps
needs to be transformed into some number of inches along the x-axis.


(2) If I tell students to "plot current versus voltage," or my preferred way is to say "plot current as a function of voltage," many will ask which do I mean is the x-axis, etc. Those who don't ask seem to have a 50/50 chance of doing it backwards.

Am I old fashioned?

Old-fashionedness is not the issue AFAICT. It's just terminology. It's a matter of convention. Students weren't born knowing the terminology or the conventions.

We should give the students some sympathy, because the terminology
is inconsistent and sometims abusive. When plotting y versus x, it is not necessarily true that y is a _function_ of x. Example: In the context of magnetic hysteresis, it is OK to say we are plotting B versus H, but strictly speaking B is not a function of H (nor vice versa).
http://www.irm.umn.edu/hg2m/hg2m_b/Image10.gif


If I use the phrase "plot current as a function of voltage" would most (all?) practicing physicists understand I am askling for current on y and voltage on x?

Well, I would understand ... but this understanding is based
on experience as to what is conventional /in this context/, not
on a-priori obviousness, and not on the dictionary meaning of
the words.

I emphasize that this is context-dependent and therefore not
knowable in advance. For example:
-- In high school, motion is plotted as x versus t with the t axis conventionally horizontal.
-- In a junior-level modern physics course, spacetime diagrams
conventionally have the t axis vertical and the x axis horizontal.


If so, why aren't students learning this?

Well, they can learn it ... but they weren't born knowing it.


If not, what terminology should I be using if I want to use what's being practiced?


If we want a plot with current horizontal and voltage vertical, I
would just tell them to plot current horizontally and voltage
vertically. They say I have a keen grasp of the obvious.


(3) What alternative words do we have for a 3-D coordinate system? For a standard right-hand system I generally say the x-axis runs left to right (right positive), the y-axis runs backward to forward (forward positive), and the z-axis runs bottom to top (top positive).

I know axes can point anyway you want them to point, but do most people generally visualize it as I said? In textbooks we often see x to the right, y pointing up, and z pointing backwards. To me, that does not fit with other practical things such as numerically-controlled milling machines, etc.


You think that is bad? How about aeronautical engineering, where the
+Z axis conventionally points /down/?!
http://www.av8n.com/how/htm/motion.html#fig-axes

This is all a matter of convention. What's conventional in one context
is unconventional in another.


Bottom line... if there is some standard language that is different than what I visualize, I have missed it, and so have many years of students. If there isn't any standard language, or if the standard is not being taught, no wonder students are confused.


That's my bottom line: It's no wonder that students are confused.

My advice: Cut 'em some slack. Tell 'em we realize they weren't born knowing this stuff, it's a matter of convention, the conventions are context-dependent, and today we're going to do it "this" way.

_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l