physics homework

[gwnn]

I'm working as a teaching assistant this semester. Mainly what I do is I have to correct 30 homeworks each week. I've found myself in the following absurd situation:

 

There is a problem that is worth 4 points. Almost everyone got point a) right using exactly the technique the professor had in mind.

 

For point b) almost everybody (8 out of 9 so far) used some strange alien formula. It led to an odd and impossible result in the case of 5 of the students so I just gave them 0 out of 2 points. However, 3 of them used the alien formula with a slight twist and got the exact result!! Nobody named the formula so I can't confirm its veracity. So if I gave 0 points to the people who got the wrong answer with the wrong formula, what do those people deserve who got the right answer with the wrong formula?!?!

 

Technical part:

the problem is to calculate the refractive index of air at a certain pressure (544 mmHg as opposed to the normal 760mmHg) with the aid of certain data obtained with an interferometer. The thing is that the pressure is not relevant at all for this problem, my professor just put the values in to give an idea of what domains we're talking about. So these 8 students used the air pressure data as well in the alien formula. The refractive index in the problem for 760 mmHg is 1.0003; 5 students got the refractive index of 1.00055 (obviously absurd since it should be lower for lower pressures) and 3 got the expected answer of 1.00022. The ninth student solved it correctly with the technique we expected.

 

Please help me out, I don't know what right and wrong is.

[nigel_k]

You need to investigate more. The formula can't be that strange if nine students used it and they didn't all copy someone's answer because they got different results.

 

But I would be inclined to give full marks to anyone who got the right answer and only look at their methods for the purpose of giving partial marks to people who got it wrong.

[helene_t]

According to

http://www.zemax.com/kb/articles/223/2/How...odel/Page2.html

 

n-1 is proportional to the pressure.

 

Was that the formula they used?

 

Edit: HotShot's reference is probably better.

[hotShot]

Well google comes up with:

 

http://www.kayelaby.npl.co.uk/general_physics/2_5/2_5_7.html

 

 

ntp − 1 = (ns − 1) × { p[1 + p(60.1 − 0.972t) × 10−10] } / { 96 095.43(1 + 0.003 661t) }

[hrothgar]

Right Answer, wrong formula should be a zero

 

However, you had better be damn sure that this really is the wrong formula...

(If there is an alternative way to structure the problem and you've missed it you're going to get crucified)

[helene_t]

well the problem is that the student were supposed to compute the refraction index from the data points but they chose to compute it theoretically instead. So it's not quite clear if it counts as right or wrong.

[gwnn]

I've looked carefully at the course manual and there's no sign of any similar formula. They definitely did not hear it at the course either. Well I will be crucified no matter what, so wtp? Anyway out of 27 students, 18 used the alien knowledge; creepy.

 

Sure it would be rewarded maximum if someone had written "I read on wikipedia that 1-n ~ p so then plugging it in we get .... " etc but just using a weird formula?

[jdonn]

Call me cynical but it sounds like the students are cheating, or are they allowed to copy off each other's assignments? It's unthinkable that 5/9 students would by coincidence use a formula you hadn't been taught in class and gotten identical answers with it, whether the answer they got is right or wrong.

[hotShot]

It is standard scientific practice that any formula or method you use,

 

a] has a name and is referenced in every decent textbook (by definition the course manual is part of this set...) or

 

b] has a reference pointing to the source, where it comes from.

 

Since the formula is not of type a] and has no reference b], you can reduce the points by any amount you like.

 

Obviously the students did not understand the question and one of them found some sort of solution that was rumored around.

[gwnn]

Students are allowed to cooperate but it's not really encouraged. In practice it helps my grading since these study groups usually hand in their assignments next to each other in the pile and they're almost always identical and I just need to skim through them. This extraterrestrial formula appeared in 66% of the papers, obviously in more than one study group, it's really mindboggling.

[blackshoe]

In my tenth grade trigonometry class, where the topic was trig identities, I was called up to the blackboard to prove a particular identity. I was called, I'm sure, because I was paying more attention to the book I was reading than to the teacher. So, I looked at the equation for a minute, and said "well, if you take the derivative of both sides, multiply by <something or other, I forget>, <do something else>, and then integrate both sides, you get cos X = cos X (or some such - it was a long time ago)". The teacher's reaction? "You can't do that! You don't know that yet!" :)

 

The book, btw, was Thomas' Calculus and Analytic Geometry.

 

That said, if the purpose of the exercise was to learn (or demonstrate knowledge of) how to use empirical data to arrive at some value, then I would give no points to those who just used some theoretical formula. I might give extra credit, though, to someone who found the answer from the data, and then used the theory to confirm.

 

Heh. I just remembered another amusing story - I was tasked, in an undergraduate course, to replicate the Michelson-Morley experiment (not exactly the way they did it, but using the same principles). My calculations led to the conclusion that the speed of light is approximately 29 mph. Since theory - or rather the results of the original experiment - told me that was ridiculous, I redid the experiment and found my error. As I recall, I got full credit for that one. :P

[cloa513]

That brings me back to marking naval engineering assignment papers- students were meant to simply add up the area of hydraulic moment on a graph however some did the full integration of the descriptive equation- it made little difference to the result except that full integration increases the chance of making an error. Indeed many did question answerring in considerable different ways from the coursework. Not wise to give extra data if you don't wish students to come up with other solutions. See if the formula works for other cases, alternatively can't you just ask one student where they got it from.

[y66]

I once got full credit from a respected prof for an answer I arrived at by inspection, which I stated, even though I clearly didn't know how to solve the problem using the stuff he was teaching. I didn't get to many of his classes but he never held that against me.

 

Agree with others that correct answers arrived at using a nonsensical thought process get zero credit and, conversely, incorrect answers due to typos when using a sensible thought process get proportional credit. And that knowing which is which is what grad students get the big bucks for.

 

Trust your instinct and don't worry about it. You can correct a mistake if you're wrong or just make a note on their paper telling them to explain their answer if they want full credit.

[Cascade]

When I was a graduate student I got the job of marking roughly 200 undergraduate assignments.

 

The Professor handed me some solution and a marking schedule and left for an overseas conference.

 

The assignment specifically required the use of a particular method to solve the problem - I forget now exactly what. The students knew other tools that would or at least might be used to solve the same problem. The question clearly said "Use method ABC to solve:".

 

Out of 200 papers less than a handful used the prescribed method. And very few of the rest managed to obtain anything like a correct answer with any other method.

 

After grading the assignments the average mark was less than 10% with the mode being 0%.

 

When the Professor returned I was in trouble - "you have to give them some encouragement". Although I didn't really feel very guilty about it - I am not sure why they didn't use the proper method maybe it hadn't been taught very well. I explained that almost the entire class made no attempt to answer the question as written.

 

However if your Professor is available I would consult with him/her before making any harsh decision yourself.

[matmat]

n = 1 . wtp?

[OleBerg]

In my tenth grade trigonometry class, where the topic was trig identities, I was called up to the blackboard to prove a particular identity. I was called, I'm sure, because I was paying more attention to the book I was reading than to the teacher. So, I looked at the equation for a minute, and said "well, if you take the derivative of both sides, multiply by <something or other, I forget>, <do something else>, and then integrate both sides, you get cos X = cos X (or some such - it was a long time ago)". The teacher's reaction? "You can't do that! You don't know that yet!" :unsure:

 

The book, btw, was Thomas' Calculus and Analytic Geometry.

 

That said, if the purpose of the exercise was to learn (or demonstrate knowledge of) how to use empirical data to arrive at some value, then I would give no points to those who just used some theoretical formula. I might give extra credit, though, to someone who found the answer from the data, and then used the theory to confirm.

 

Heh. I just remembered another amusing story - I was tasked, in an undergraduate course, to replicate the Michelson-Morley experiment (not exactly the way they did it, but using the same principles). My calculations led to the conclusion that the speed of light is approximately 29 mph. Since theory - or rather the results of the original experiment - told me that was ridiculous, I redid the experiment and found my error. As I recall, I got full credit for that one. :unsure:

Wouldn't it have been easier to broaden the definition of "approximately"?

[hanp]

In general I'd say that unless we have specified the method they should use, students should be allowed to use any method they wish. However, if a student uses a formula that hasn't been taught in class, without saying where the formula comes from, and the formula is also incorrect, then I would not give any credit.

[pooltuna]

I've looked carefully at the course manual and there's no sign of any similar formula. They definitely did not hear it at the course either. Well I will be crucified no matter what, so wtp? Anyway out of 27 students, 18 used the alien knowledge; creepy.

 

Sure it would be rewarded maximum if someone had written "I read on wikipedia that 1-n ~ p so then plugging it in we get .... " etc but just using a weird formula?

It seems harsh to punish extra research seeking to find a solution to a problem. This is something new to physics from my day when researching alternatives to book solutions was virtually impossible.

[Fluffy]

we lack critical info for taking a decision: is there any beatiful icelandic blonde among the students whoom you could take for private lessons?

[gwnn]

I've looked carefully at the course manual and there's no sign of any similar formula. They definitely did not hear it at the course either. Well I will be crucified no matter what, so wtp? Anyway out of 27 students, 18 used the alien knowledge; creepy.

 

Sure it would be rewarded maximum if someone had written "I read on wikipedia that 1-n ~ p so then plugging it in we get  .... " etc but just using a weird formula?

It seems harsh to punish extra research seeking to find a solution to a problem. This is something new to physics from my day when researching alternatives to book solutions was virtually impossible.

it does seem harsh but why don't they say where they got the formula from?

[matmat]

it does seem harsh but why don't they say where they got the formula from?

There's always the possibility that they encountered this formula somewhere in their prior schooling; either in a chem class or an earlier physics class. They should still mention where it came from, but if it is something they saw not too long ago elsewhere, they might not feel the need to cite.

[kenberg]

I was a grad student in th sixties, a professor for quite a while. I have a couple points.

 

1. Ask the prof. what he would like. I think it is impossible for us to say because wwe did not read the problem. It souinds as if the point of the problem was to use data to get the result supported by the data. If that was the point and, important, it was clearly stated that this was what was wanted, then using any theoretical formula is not waht they were asked to do. There is theory, and there ar experimental results.

 

2. This concerns the working together versus cheating aspect, and I regard it as far more important than the problem. Some years back I was on a review panel for physics students accused of cheating on their lab report by working together. As you say, some working together was encouraged. The students' defense was that they had not at all disguised their common work and they were doing what they thought they were supposed to do. Some inquiry revealed that the professor, the graduate student, and the lab manual gave three substantially differing accounts of what proper practice was. The students' understanding seemed to be in reasonable conformance with one of these accounts. Of course they were judged to not be cheating. This led to some hard feelings about letting students get away with things. Lab reports are frequent sources of such problems. If there is any ambiguity at all in what is and is not allowed I hope it will be clarified immediately.

 

I will perhaps recite a couple of my favorite stories later, but dinner is on the table and my wife is not so patiently awaiting my presence.

[gwnn]

The formula appears to be correct after we have looked at it for about 15 minutes with my supervisor. The most likely explanation seems to be that someone found a similar problem online and just copied the final formula from it and plugged in our numbers. Then this formula spread around and two thirds used it finally. A problem like this has already been discussed and solved in a seminar before and nothing similar to the alien formula was ever written.

 

technical part: They used the idea that n-1 ~ p and did not use the refractive index given (n=1.0003 in 760mmHg).

[matmat]

technical part: They used the idea that n-1 ~ p and did not use the refractive index given (n=1.0003 in 760mmHg).

Lol... how did they get the prop. coefficient?

[Zelandakh]

Pick a random student at the start of the next class and ask them to solve a similar question on the blackboard alone. Chances are they will have simply copied the formula and will have absolutely no idea. Allowing this to continue is very bad practise as it encourages the students not to actually learn which tends to be bad come the final exam. If there is no final exam (coursework assessment) then it is even worse since you are not giving a fair mark to all students.