try square question

Interesting. Lots of poor education practices exist in all sorts of places, often characterized by rote memorization and lack of actual thought.

I think you misinterpreted what I said, or I didn't explain clearly.

Whoa, abstract and practical are the same? I hated geometry in high school because it was never clear how to get from one concept to the next. One of the reasons was that the basic definitions were not stated or emphasized so the student tended to start off in water where he couldn't put his feet down. Another problem was the confusion over abstract and practical definitions of a line. The abstract mathematical concept of a line is that it can't be seen, has no beginning and no end, is continuous, and has no width.

As for mathematical definitions of angles, the definition is the inclination of one line to another (thus two lines are required for an angle). When they intersect at 180 degrees it is called a "straight angle." Now that's a real oxymoron! If the angle is 360 degrees it is a "perigon" while 0 degrees is called a "zero angle." These terms may be useful in some mathematical disciplines but 360 degrees and 0 degrees are the same and "zero angle" is another oxymoron. I suppose it is easier to say zero angle rather than day there is no angle. If you apply the abstract mathematical concept of a line, i.e., without beginning and end, to these angle definitions, then there really is a single line when two intersecting lines form a "straight angle" or "zero angle." (note we are assuming all lines lie in the same plane.) All of which has essentially no practical value to woodwork but I will continue to assume that my straight edge is a single line and does not include a straight angle.

Bill Rogers wrote:

The measure of angle has nothing to do with the measure of length, except as inferred by ratio in trig relations. Your use of "a second line is rotated on the same axis to 180 degree from the fixed line" is inconsistent with your " the rotated line is no longer at an angle to the original" You can't have your cake and eat it. The measure of that angle is 180 degrees, period. If you rotate it 180 degrees, it forms a 180 degree angle, even though it becomes collinear with the stationary line segment. In fact, "collinearity" is the key here.

When you use a try-square to make your jointer table guide at 90, do you say you can't do it because when it lines up the space between disappears? Or is it that both angles [the try-square and the fence-to-table] are now equal, being 90, and so adding to 180?

Bill.

Sorry about that, but it *was* clear to others. I could lead you precisely from one to the next [**]. That's the whole point of it, the logical development. Part of the problem is that you could be "given" the properties and try to commit them all to memory [meaningless if there's no connection], or you need to develop them and see where they come from [theory], and so understand them and their interconnection. You might then find even better uses for them.

It's called "completeness". The number system was invented [not visible anywhere in the 'real world' people refer to.] The number Zero was invented. It was necessary. It does not denote "nothing", but is a symbol in counting just as good as is the symbol for other quantities. What would you do instead without it? As simple as it is, it's one of the great inventions of the mind. ["I saw a man upon a stair ..."]

A "straight angle" is not an oxymoron. It is still an angle ...a "straight" one. It's still there; we can just now give it another name as well suited.

By itself, a hammer is useless. It needs your skill to use it. By itself math is useless, just a game of the mind. It also needs your skill to use it. One of my favourites: IKEA designed a table, useful in B&Bs for example. Take the top and rotate it 90 degrees, and flip it on the dividing hinge, and you have a top twice the size. Where do you put the pivot point so that it will sit square, centered on the base. Second question: What shape [ratio of sides] must it be to keep the same rectangular shape when opened up?

Hating math is like hating a hammer. Don't hate it, use it. It took a whole lot of people over 3000 years to develop.

P.S. You can use your try-square to make sure everything in the table lines up. :-)

Bill.

Sounds like you had a really incompetent geometry teacher.

The thing to understand about math is that it's a game, it has rules, as long as you obey the rules anything goes.

With geometry, you have rules that are called "postulates" and "definitions" and given those the game is to create "theorems" which are "proven" based on the logical development from "postulates" and "definitions".

Going in a different direction, with compass and straight-edge you do "constructions" of various shapes--these, before the availability of accurately marked scales and protractors were of great importance in drafting--since constructions are necessarily done using real approximations of the ideal it is best to always remember this and provide a check of one sort or another on each construction, as for example by repeating it several times and taking the centerpoint of the resulting group of marks.

Your geometry teacher should have made that clear to you at the outset. If he did then you'd probably have had a lot less trouble with it.

Sounds like you got all your math from this same teacher. "Perigon" appears to be one of those wonderful terms that was created during the Victorian era, probably in an effort to standardize terminology (if the distance around a circle is the "perimeter" then the angle subtended by the "perimeter" must be the "perigon" . . .), and its utility is that if 360 degrees is a "perigon" then 180 degrees is a "hemiperigon" and 90 degrees is a "semihemiperigon" and so on. Might still be used in the UK but it's certainly not used in mainstream math, science, or engineering in the US.

That is a perfectly reasonable assumption since a straight edge is not used for measuring angles unless it is attached to a protractor.

It's actually called a straight angle, which makes no sense at all. a

I don't understand what you are saying. Are you talking about the fence being set up at 90 degrees. If so, when you draw this the vertical line indicating the fence and the vertical line of the try square would be a single line. And the horizontal line indicating the face of the jointer and the horizonal line indicating the horizontal leg of the try square would be a single line. You still have an intersection of 90 degrees between the jointer face and the fence as well as 90 degrees between the legs of the try square. Maybe I misunderstood you, but I don't see your point. Or are you confusing lines on a paper and the representation of solid objects in real life.

Whoa, you are infering a lot of things I never said. I didn't say I hated math, I said I hated geometry (hate is a little strong) and really implying that I hated the way it was taught. The terminology in most fields becomes a little ridiculous, and simple normal words will do just as well. Those with lots of education in the field often use the more complex terminology just to impress the less learned. But some of the terminology is down right bad as it implies things that were later found to be untrue. In the practical world, zero does mean nothing. I've got zero dollars means I have no dollars. Nothing is and always has been a clear concept in the real world. And having grown up while zero was a concept in math, I have a hard time understanding how it took so long to develop. As to a hammer being useless without skill, tell that to a stone age man. How much skill did it take to bash another man's head with a rock.

Math isn't a game. Math is a language that tries to describe the real world. As man's knowledge of the real world increase, math has had to change and develop more complex words. Geometry has it uses but it is pretty primitive and is useless in describing the more complex phenomenon that we consider common knowledge nowadays.

It's the kind of thing that comes up in formal proofs.

"Prove that points A, B, and C are colinear"

The usual approach would be to construct a few line segments, then talk about them using formal language. Here's the first and last lines of such a proof:

Suppose, on the contrary, that points A, B, and C are NOT colinear, then angle ABC with vertex B . . .

(lotsa other stuff)

. . . meaning that angle ABC is a straight angle. Therefore points A, B, C are colinear.

But I agree with you that this is something that won't come up much in ordinary conversation:

"Hey, Fred! Pass me that straight angle. I need to cut this Sheetrock"

Most in highschool are incompetent, but it could have been the student also.

I found geometry all rather a waste of time and a useless exercise, development since Euclid made most of the theorems rather common knowledge. Now trig was of interest because you could actually use it for something.

Exactly. The advancement in tools makes most of geometry rather retrograde. I've learned how do to square roots by division several times and then promptly forgot how, and learned to do square roots by subtration of 9s or something like that on old electric calculators before computers, but I certainly don't wouldn't do that anymore. It's as quaint as building a canoe by hollowing a log.

Lots of math teachers don't make things clear. The silly word problems were a constant problem for most students, in simple math, algebra, chemistry, and physics. They new that you did something with A, B, an C, but weren't sure whether it was A/B xC or A/C x B. Like most students I hated them. It wasn't until I started teaching chemistry and making up my own word problems that they became rather fun, but I was always clear with my students that word problems were puzzles that required a thought and not mechanically adding, dividing, etc. I say silly word problems because most contained the exact number of data to find a solution. When you introduce needed and useless data into a problem, and tell the student that a graph or some sort of depiction may help, the student can start understanding.

Not hardly from the same teacher. And my point was precisely that many terms are archaeic and inappropriate for the present day.

Terms like "perigon" and "straight angle" aren't taught in most math curricula--"straight angle" I could figure out but "perigon" I needed to look up, and I used to get paid for that sort of thing.

Now I know I've been told in my lifetime I'm as "thick as two short planks", but bear with me and I'll try to convince you.

Which is ..."the measure of rotation of one line [segment] to another ...which can also include 180. When the segments line up, they can be called a single line.

Thinking backwards... every line is a set of points, and all you need to do is to choose one to form line segments, which are then at an angle [of 180]. You can then freely rotate one with respect to the other forming other angles. So decrease it [from 180] to get other angles less than 180, or increase it. But 180 fits right in there! The finite number system, and the geometry is continuous. There is no gap.

Fence to table = 90. Fence to try-square = 90. when they meet [90 +

Bill.

What simple words for example? What do you call "normal"?

I do understand, you know. I'm from a blue-collar background, and love working with my hands. But you shouldn't knock what others might find useful even if you don't. Some fairly complex geometry and other math is needed to produce the toys we take for granted, and what we call a normal part of our everyday life. It really does require some fairly hefty definition at times. A "chair" or a "seat", it's just something to sit in and relax and smell the roses.

Peace.

Bill.

Bill.

The formal part provided a basis for understanding the informal results. It also taught to not rely on assumption, but granted that was an exercise in logic rather than the properties of shapes.

I use geometric constructions when I want dead-on accuracy, or want to form a shape not doable otherwise nearly as efficiently. If I want dead-on 90 degrees, 60 degrees or other variations I use compasses and straightedge [the 180 kind ... :-) ]. Lots of other similar uses also. Perhaps I should write the book using the KISS principle?

I can't tell you the number of times friends who are pro carpenters tell me they really shoulda listened! Now they are using geometry [including Pythagora theorem!] every day. if you want a REALLY good book, get "the Carpenter's Square" and learn how to do things efficiently, including building a spiral staircase.

Just on that point, it's tough enough to teach youngsters what is to them complicated algebra. Those "word problems" are the simplest applications dreamt up. If you can make up a textbook full of simpler applications not using "word problems" I'd have been the first to buy it. The only other alternative is some "real world" problems, usually well beyond the young and inexperienced. We see it better only in retrospect.

Bill.

Now thats when Fred find that no skill is needed to effectively swing a hammer. I wonder if there is a flat arc?

Botanists use the term leaf "margin" whereas you could more simply say leaf "edge." A bit more technical is to talk about a meristem, which means nothing to most people, but you could just say the growing point. I'm not knocking useful terms, hell, I'm a botanist and use technical terms as appropriate and technical terms are expected among peers, but use of technical terms to a non-technical audience when a common word will do is silly and destructive to communication. And we are all a non-technical audience in some field. An example that I especially like is the use of siphon by hydraulic engineers in the description of water ways. The fact is that the structure is just a pipe with a curve that starts high and end at a lower elevation (like a J). True siphons in water ways are extremely rare, both because filling the siphon to start the water flowing is difficult or expensive and most conveyances would collapse at the high points due to the negative pressures. Stand around and listen to a technical discussion in a field that you know little about and soon you will be saying, "Why didn't he just say.......?"

Yeah. I used to have the same problem with botany. :-)

Still and all, it's been an interesting discussion. Most of all it's interesting to see those who know a little trying to say a lot [no personal reference there!] That's why I keep my mouth shut when around botanists talking about botany. I thought Krebs Cycle was something he used to ride to work.

Bill.