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Forms of Quadratics

You can represent a quadratic equation in a whole lot of different forms. Each has its advantages.

Standard forms let you learn valuable information about an equation just by looking at what number was put where. There are three common forms of quadratic equations, also known as second-order polynomials.

Note that variables like a, b, and c in these descriptions will change in meaning from form to form.

Standard form

$$y=\text{ax}^2+\text{bx}+c$$

Advantage: Relative mathematical simplicity, sets you up to use the quadratic formula, and easy to find the y-intercept. It is just the constant y=c.

Example: $$y=2 x^2+12 x-14$$

Factored form

$$y=a(x+b)(x+c)$$

Advantage: Easy to find the x-intercepts. They are at x=-b and x=-c. You don’t even need the quadratic formula!

Example $$y = 2(x+7)(x-1)$$

Vertex form

$$y=a(x-h)^2+k$$

Advantage: Easy to find the vertex, which is at (h,k).

Example: $$y=2 (x+3)^2-32$$

All three of the examples from this post were actually the same quadratic equation! See if you can identify the vertex, x-intercepts, and y-intercepts using the different forms.

K: The Stress Intensity Factor

Fracture Mechanics is the study of how cracks grow and materials break apart. One of the most important concepts in the world of fracture mechanics is K, the linear-elastic stress intensity factor.

It is easy to confuse what K is used for with what it actually means. K is useful for measuring a material’s resistance to fracture starting at a sharp-tipped crack. However, K is defined as a measurement of the magnitude of stress distribution around a sharp crack tip in a linear-elastic solid. K only has meaning in the context of materials that can be expected to form a certain distribution of stress around a sharp crack tip!

So if we have something that is predominantly linear-elastic (high-strength steels, glass, etc.) and it contains a sharp-tipped crack, then the stress distribution around that crack can be given in terms of this parameter we call K. The stress distributions for a crack in tension perpendicular to the crack faces (aka Mode I loading) are:

$$\sigma _{\text{yy}}=\frac{K
}{\sqrt{2 \pi r}} \cos (\theta ) \left(\sin \left(\frac{\theta }{2}\right) \sin \left(\frac{3 \theta }{2}\right)+1\right)$$

$$\sigma _{\text{xx}}=\frac{K
}{\sqrt{2 \pi r}}
\cos (\theta ) \left(1-\sin \left(\frac{\theta }{2}\right) \sin \left(\frac{3 \theta }{2}\right)\right) $$

$$\sigma _{\text{xy}}=\frac{K
}{\sqrt{2 \pi r}} \left(\sin \left(\frac{\theta }{2}\right) \cos \left(\frac{\theta }{2}\right) \cos \left(\frac{3 \theta }{2}\right)\right)$$

These plots show how the stress distribution looks for a stress intensity factor of K = 1 MPa √m.


Stress distribution for a stress intensity factor of K = 1 MPa √m. The crack tip is located at (0,0) and the crack follows the x-axis along x < 0.

Stress distribution for a stress intensity factor of K = 2 MPa √m. Note that I kept the same scale so that we can see the effect on the spatial distribution.

Hopefully, this helps illustrate what the stress intensity factor, K, means in a mathematical sense. Though K is useful for measuring fracture toughness, it is only applicable for sharp crack tips in linear-elastic materials that exhibit stress distributions similar to what I have shown here.

Note that these solutions are approximations: they are the first, most significant term of a summation. But K is still a very useful parameter for materials that obey linear-elastic fracture mechanics.

I created a live-view applet for Wolfram Cloud, but it wasn’t playing nice with the cloud, hence my screenshots above. In the event I get it working, I’ll put it here.

Ce qui est simple est toujours faux. Ce qui ne l’est pas est inutilisable.
What is simple is always wrong. What is not is unusable.

Paul Valéry

Silly Math Question: Is It Possible to Write Pi Backwards?

This was inspired by the Quora question “What is Pi backwards?” It seemed like a good early post to try converting Mathematica code and LaTeX to WordPress.

The concise answer to the question is “That question is meaningless,” but why is this the case? After all we can write 123 “backwards” as 321, why not π?

One explanation is that π is an irrational number with unlimited digits, and something without an end cannot be reversed. Or something with no end, written backwards, becomes a thing with no beginning. Or one could say that “reversing” any number really doesn’t have much meaning in the first place. Yet, this question gives us a good opportunity to explore what we mean when we describe π as an irrational number.

One way to look at π is as a summation, like this summation equivalent to the arctangent function. $$4*ArcTan(1) = \pi$$

$$4 \sum _{n=1}^{\infty } \frac{(-1)^{n-1}}{2 n-1}=4 \left(1 \frac{1}{1}-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}\text{…}\right)=\pi$$

If you look at that sum, it adds up an infinite terms, and every term is smaller than the previous one. Thus, every term we evaluate brings us closer to the actual value of π. But, we never run out of terms, so we never find the exact value of π. We just get closer and closer forever.

We can approximate π by going out to 100 terms.

$$4 \sum _{n=1}^{100} \frac{(-1)^{n-1}}{2 n-1} = 3.13159$$

This gets us in the neighborhood. (Note that I rounded to 7 digits.) But we can get closer by going to 1000 terms.

$$4 \sum _{n=1}^{1000} \frac{(-1)^{n-1}}{2 n-1} = 3.14059$$

Now π is starting to look like the number we recognize! If I kept adding more terms, then I would need to round to a larger number of digits to see the change. If we could go forever, the summation converges to π itself. Now you probably have some idea of why approximating π to a certain number of digits is such a popular problem to throw at big computers.

This sets us up to see why “π backwards” ends up being meaningless. The number has no end, so there is nowhere to “start” the reverse-order number. Just for fun, though, we can interpret “π backwards” as an excuse to switch the starting and ending place of the summation.

$$4 \sum _{n=\infty }^1 \frac{(-1)^{n-1}}{2 n-1}=\pm 4\left(\frac{1}{2 \infty -1}-\frac{1}{2 \infty -3}+\frac{1}{2 \infty -5}\text{…}\right)$$

Notice that we are starting at infinity, so we will never reach a definite answer to this summation either. But it’s worse than that. We are effectively dividing by infinity in every term, so every term is practically zero. We can’t even figure out if the “first” term is positive or negative! (Not that it really matters when every term is zero.)

$$4 \sum _{n=\infty }^1 \frac{(-1)^{n-1}}{2 n-1}= \pm 4\left(\frac{1}{\infty }-\frac{1}{\infty }+\frac{1}{\infty }\right) =\pm (0-0+0\text{…})$$

So we do have an answer for what “Pi backwards” might look like. Every term is effectively zero, yet infinitesimally larger than the previous term, we can’t even figure out the sign of the terms, and there is an infinite number of these infinitely insignificant terms to add up. We will never get to a term that isn’t zero because by starting at the “infinity” term, we put ourselves infinitely far away the “end” of our summation.

So “Pi backwards” looks like an infinite summation of zeroes. At least looking at π forwards gave us a reasonable approximation of the actual number because it prioritized the largest, most significant terms.

Film is one of the three universal languages, the other two: mathematics and music.

Frank Capra

A Blog is Born!

Welcome to my new website! I am Wade Lanning, a materials scientist, lab manager, and teacher. This will be my platform for sharing my various projects and lessons, and hopefully, this post is the first of many!

So get ready for a stream of whatever I happen to be doing any given week! Right now, I am working primarily as a tutor, and most of my posts will be based on lessons I prepared for my students. However, you can also expect snippets of my past research, miscellaneous mini-projects, photos, or whatever I happen to be working on that day.

Progress lies not in enhancing what is, but in advancing toward what will be.

Khalil Gibran