# What do we mean by nonlinearity in physics? ('cause wikipedia sucks)

1. Dec 6, 2013

### piareround

Physics teacher and physics professors need to come up with a better definition of nonlinearity if we expect our students to pass any basic physics course. This is because of how education has changed over the past 10 years. I hope in this thread, we collectively as members of this forum can come up with a better definition.

So I was learning about Follett's Information Skills Model from Pathways to Knowledge in education class, when I came across this sentence:
http://www.intime.uni.edu/model/information/proc.html
http://www.k12.hi.us/~mkunimit/pathways.htm
The problem is outlines of the model itself are very linear and cyclically linear in nature at least based on comparing to examples of linearity and nonlinearity in physics. This is mainly because other than in literature, I know no other examples of nonlinearity other than in physics. This got me thinking, how do we really define nonlinearity in physics? Is really that different from how other professions definite it?

However, when I took a quick glance at Wikipedia I found the worst definition for nonlinearity possible:
http://en.wikipedia.org/wiki/Nonlinearity
I really don't like this definition because there is a lot of physics phenomenon can be broken down into more manageable parts by the super position principle or approximately obeys the superposition principle. There is also a lot of nonlinear phenomena in physics. Furthermore, sound, light, and quantum mechanics electrons in atoms itself serve as the core of how we define to student what superposition is. We use things that can approximately obey superposition to define and introduce what superposition is!! We do it before they even learn laws of mechanics, because teachers are not required to teach superposition in 6th and 8th grade!!! Superposition and waves making up a fundamental part of the new science core standards!

So, are we preparing student to fail through misunderstanding by then turning around an saying these examples of superposition are also not examples of superposition in nonlinearity?

I don't think we are preparing students to fail in such a way nor do i believe that we are trying to lie to students. However, the potential for a logical reason for failure is there. Thus, I feel that using superposition as only test of nonlinearity itself, is a poor definition and way of teaching nonlinearity.

So can anyone think of a better physics definition of nonlinear than how Wikipedia defines it? What do we mean by nonlinear in physics?

P.S. The mathematics definition is a bit better of course; however, I am not asking about that. I only post it to show how much in contrast it is to the nonlinear definition they give for it in physics. What I am asking about is for a better definition of nonlinearity in physics not mathematics.

Last edited: Dec 6, 2013
2. Dec 6, 2013

### Simon Bridge

"A system that is not linear" is the best definition of a non-linear system you will get. Sorry.
Instead of trying to define a negative - why not look up the definition of "linear"?

The physics definition is the same as the math definition.

3. Dec 6, 2013

### Bobbywhy

One good starting point is our old standby, Wikipedia:

"In physics and other sciences, a nonlinear system is the opposite of a linear system, that is a system that does not satisfy the superposition principle, which means that the output is not directly proportional to the input.
http://en.wikipedia.org/wiki/Nonlinear_system

4. Dec 7, 2013

### stevendaryl

Staff Emeritus
Linear or nonlinear is about a relationship between two variables. For example, suppose you have a spring hanging from the ceiling. You connect a weight to the end of the spring, and the length of the spring will increase. Then the relationship between weight and length change is linear if the change in length is proportional to the weight added. You put a 1 pound weight on the spring, and the length increases 1 inch. You put a 2 pound weight on the spring, and the length increases 2 inches. Etc.

Mathematically, the relationship between two variables $X$ and $Y$ is linear if there is a constant $C$ such that $X = C \cdot Y$.

5. Dec 7, 2013

### Claude Bile

Nonlinear means not-linear; I don't see how it could be explained any simpler than that.

I don't understand what the problem is.

Claude.

6. Dec 7, 2013

### AlephZero

I think the problem is confusing psycho-babble with science.

Note, the "pathways to knowledge" quoted by the OP should have a "registered trade mark" symbol after it. You don't often find those symbols in real science, in my experience.

7. Dec 7, 2013

### dipole

There are many different kinds of non-linearities, while there is only one kind of linear. So you won't find a definition that exactly describes every scenario - however like everyone else I think the current meaning of the term "non-linear" is perfectly clear to anyone who studies science.

Might wana read into a subject a bit before you waste so much effort going on long tirade in the future.

8. Dec 13, 2013

### Claude Bile

Aye, the day that physicists change definitions to suit psychologists is the day I give the game away.

Claude.

9. Dec 13, 2013

### dauto

The original poster ought to better explain why he doesn't like the Wikipedia definition. To me it seems to be clear, correct and concise. What else could one ask from a good definition?

10. Dec 13, 2013

### Simon Bridge

I'm with dauto - we need to hear back from OP to make sense of the question.
I understand OP is an 8th grade TA ... so we'd probably benefit from knowing the education level needed here too.