# Physics HW FAQ Discussion

1. Apr 17, 2005

### Gokul43201

Staff Emeritus
This is space to start writing up and discussing ideas, structure and content for a possible Physics Homework FAQ type of sticky thread.

Note : This is an attempt to implement marlon's idea in the Feedback Forum

Last edited: Apr 18, 2005
2. Apr 17, 2005

### Gokul43201

Staff Emeritus
Let me start this rolling : I think most of the following questions belong in the K-12 section. Here are a few :

1. What is a "good" approach to solve a typical kinematics problem ?

(equations of motion under constant accel - average velocity - cases when a = 0 - graphical representation - some examples - relative velocities - addition/subtraction of vectors - some examples)

2. What is a "good" approach to solve a typical Newtonian mechanics problem ?

(free body diagram - resolution of forces - how friction works - resultant/net force - Newton's second law - when to use conservation principles and which ones to use when - some examples)

3. Kinetic theory of gases, anyone ?

(gas laws - postulates of the kinetic theory - how to think of temperature in terms of mean kinetic energy - velocities, molecular weights, etc - some examples)

4. How does one think about heat and related problems ?

(energy conservation - specific heat capacity : solids/liquids/ideal gases - latent heats - some examples - heat transfer : conduction/convection/radiation - thermal conductivity/newton's law of cooling/Stefan- Boltzmann equation - examples)

5. Geometric and wave optics ?

(laws of reflection/refraction - refractive index - total internel reflection - mirrors and lenses - sign conventions (or how not to follow them) - interference - diffraction - some examples)

I"m tired, but there's more topics, like sound, hydrostatics, (hydrodynamics ?), and the basics of electrostatics, electrodynamics.

Am I asking us to write a textbook ? Certainly not ! I don't imagine that any of the above topics will require more than a page, and even that's a lot. Diagrams will have to be handled through attachments.

Anyway, throw in your ideas and objections. Let's see what comes of this...

3. Apr 18, 2005

### arildno

One issue many are confused about, is the difference between material and geometric physical systems.
A material system consists of the SAME material particles over time, whereas a geometric system needs not to consist of the same particles.

The laws of mechanics are formulated with respect to a material system; if we want to to find the correct laws for geometric systems, we need to tweak the laws a bit.
In particular F=dP/dt is no longer in general true for the geometric system, since we need to take into account the possibility that new momentum (in the form of new particles) has entered our system, or that momentum has been lost in that some old particles have left the system we're observing.
A typical example of a geometric system is the rocket problem (where old mass is continually lost through fuel ejection)
Another example is whatever fluid is passing through a specified tube of interest.

Last edited: Apr 18, 2005
4. Apr 18, 2005

### marlon

Err this is not exactly what i had in mind...But, please go along...

marlon

5. Apr 18, 2005

### ZapperZ

Staff Emeritus
I do have one suggestion for this, and I would be willing to volunteer to write it (I already have some form of it written down since I have used it before when I taught intro physics).

I'd like to see something to the effect of "How To Solve Physics Problems".

I see many instances where students have very bad problem-solving technique. One such example is that they tend to plug in values way in the beginning of their work, and then get stumped when they are not given some values that they think they need. This is a major pitfall for most students learning this subject, that they somehow do not realize that they are better off solving it algebraically first. It is also easier for the rest of us to follow the work when it is shown symbolically rather than tracking what those numbers represent.

There are many others, but I think, in addition to some of the common problems being shown in the FAQ, we should also teach them how they should solve a problem.

Zz.

6. Apr 18, 2005

### Clausius2

The only way I could help would be talking about Fluid Mech (I have noticed there are sometimes questions about Hydrostatics and Bernoulli equation), and heat engines-thermodynamics.

I think most of physicists here are much better to explain basics mechanics and introduction to physics.

Let me know.

7. Apr 18, 2005

### Andrew Mason

There are many good suggestions here. I think a FAQ could contain some general things, such as Zapperz' suggestion. But I think you are going to find it to be an unwieldy project that will be very time consuming to create and maintain.

It might be better to prepare lists of model question/answers from among the archives for each type of problem and link to them separately from the FAQ page. Certain problems keep coming up over and over and it may be that students find it difficult to find them through the search facility.

AM

8. Apr 18, 2005

### Gokul43201

Staff Emeritus
Zz, I like this idea. If you could write this up and post it here, that would be a good start.

AM, I think this would defeat our philosophy of helping the students think for themselves. If students are going to have access to complete solutions, let's at least leave it be hard for them to find such solutions. I think the existence of searchable archives with problems entirely worked out over the course of the thread is in itself a (possibly unavoidable ?) drawback. It is the easy way out; we shouldn't be making it easier, should we. Anyway, that's just my opinion. Others are welcome to join in with theirs.

Marlon, this was your brainchild ! Even if this is not what you had in mind, please throw in your ideas. I thought I was providing a space to explore your idea, but if this is not it, feel free to talk about it here (if it's related) or in the original Feedback Forum thread.

Last edited: Apr 18, 2005
9. Apr 18, 2005

### arildno

I mentioned to marlon in a PM that possibly we might create a section on how to choose the most convenient/easiest description of a problem in physics.
A few examples from classical mechanics:
1) How should we place our coordinate axes to get the "simplest" description?
2) Should we choose an inertial or non-inertial frame?
3) What control volume/physical system ought to be chosen in order to simplify the arguments?
4) Conservation perspectives vs. force/torque-perspectives
Perhaps it would be illuminating to use examples where there also exist at least one other, but lot more difficult way of doing the problem, and show both ways in full.

Last edited: Apr 18, 2005
10. Apr 18, 2005

### Gokul43201

Staff Emeritus
Here's what I think we want to achieve in this thread :

1. Decide whether some form of a Physics Homework FAQ is a good idea at all.

2. If the answer to 1 appears to be 'yes', then what kinds of questions must the FAQ address : only general stylistic/approach oriented questions such as Zz's suggestion; or/and more specific (though still staying broad for the most part) physics questions as well ?

3. Make up a list of important questions to answer. And figure out how (approach and structure) we want to answer them.

4. Pick volunteers to prepare a first draft towards answering one or more of the above questions.

5. Make suggestions/criticisms and through that process amend the drafts to arrive at a final set of answered questions.

I request that we start off try to address the points in this list in each of our posts (in addition to whatever else anyone wants to say)

For my part, I think the FAQ is a good idea, but as AM has pointed out, may be a mouthful after all. I wouldn't mind taking up one or more of the questions I've proposed, if they still stand after further scrutiny.

Last edited: Apr 18, 2005
11. Apr 18, 2005

### chroot

Staff Emeritus
I believe we should follow the style of the Dr. Math program at http://www.mathforum.org. I am also a contributor there and feel it works well.

They have a pretty detailed FAQ that provides reference material -- equations, simple problem-solving techniques, fair and thorough answers to perennial questions like "Does 0.999... = 1?", and so on. Then, in addition, they provide a searchable archive of good questions, together with (sometimes several) good answers -- a sort of "hall of fame" of math instruction.

- Warren

12. Apr 18, 2005

### Galileo

I bought Polya's "How to Solve it" a few months ago and it's a real gem.
A detailed and carefully crafted book addressed to the problem of solving problems.
I can identify myself in lots of stuff he's written. I don't want to copy Polya (that'd be plagiarism), but do something similar that is specialised for the use of solving physics problems.

In my opinion, the main goal is to learn students to think structurized instead of juggling with vague bits of information here and there ("should I use this?") and some formulas ("oh, let's plug it in the equation, is this right?").
So I'm willing to write a general outline to such problems and why and when you should or shouldn't use a specific method. What model are we using and what are its limitations etc. Then a few sample problems where the solution is not only outlined, but where the thought process is written down. Usually the problem can be solved by asking the right question. By taking the student through some of these processes, he/she may get an idea. Since nothing will be learned if he/she doesn't do stuff him/herself, I think a test or quiz is a great idea too.

Those are my thoughts on the subject. I can't claim to know they will work, since Im not a teacher like some of the others here (Im still a student myself) so my experience is limited, but I aspire to be a good teacher next to a physicist.

13. Apr 18, 2005

### marlon

Guys, i think Warren made a very good point and he provided a very nice link. Like i have feared, this thread is starting to look like what we possible can do, but that's useless. Now, i realize that some discussion will be necessary but i suggest we do it like this : i have started a thread in the Homework section (i am hoping it will become a sticky) where i posted some examples of Newtonian mechanics. I have also pointed out (in the last post there or in my suggestion) which route to follow. I have started on specific things...just check it out. Now if there are suggestions and/or corrections, i think we should PM-them to each other. I know for sure that arildno is willing to contribute and he is willing to communicate through PM. I urge you NOT to communicate through posts because we will end up with what we have here : a sequence of posts that are indeed valuable to the authors but NOT to the future users of this thread. I think we should get started like i did (and let's pm-for extra texts that should be added, so we know of each other what we are doing) and continue that specific road. There is ofcourse lots of room for discussion via pm or this thread but let's keep the thread that i started to the actual problem solving posts. If someone wants to post something there (and i very much hope you will) let it be specific solutions to specific problems on the topic we are dealing with right now : Newtonian mechanics.

Besides, i also would like to make many references in that thread to posts that have been made in the homework section in the past. Like this, the users will STAY IN this forum and they won't be redirected to other sites outside this forum. Just thought the administrators would like that :)

marlon

14. Apr 19, 2005

### marlon

Last edited: Apr 19, 2005
15. Apr 20, 2005

### Gokul43201

Staff Emeritus
16. Apr 20, 2005

### marlon

marlon

ps : Gokul, could you delete dexter's post in my thread ??? thanks

17. Apr 20, 2005

### e(ho0n3

I think what arildno is doing with the post [post=538334]Homework help: Classical Systems With Variable Mass And Other Geometric Systems[/post] is really cool.

Oh, and sorry about interrupting the post like. I didn't know it had a higher purpose.

So, what does one do when one wishes to comment on or question a statement in a post such as the one above? I mean, I consider arildno's post above to be overkill for an FAQ section.

18. Apr 20, 2005

### Gokul43201

Staff Emeritus
The discussion happens here. Post your suggestions to marlon's or arilno's threads in this thread, and leave those intact.

19. Apr 20, 2005

### arildno

I would rather say I'm doing a bloody massacre of FAQ's there..

The reason why I wanted to do this particular thread, is that variable-mass classical systems are, in general, very cursorily dealt with in most introductory physics text (perhaps you understand better now why..)

I find, however, that this cursory treatment is precisely the type of treatment which easily can lead to misunderstandings of the issues involved. And I dislike that!
Thus, I felt that this particular topic ought to be treated in some greater detail..
And again, feel free to PM me if there is some particular question you don't think warrants a post in this thread.

20. Apr 20, 2005

### arildno

Thanks for the support.
I look forward to both constructive and destructive criticisms of my thread..