Thursday, March 24, 2011

My Frustration over Pseudo-"Something" Problems

I organize and volunteer for a physics help study at my university. We get a range of students, including those just looking for quick answers and help on homework, those really looking for meaningful help in learning physics, and even those who love just hanging out to talk physics.

I don't mind that some students are there to only "get' quick help and aren't in it for some deep understanding. Most of the courses aren't structured in a way to help them learn. I am happy to guide them a long a little bit, and make them a little less frustrated by the crap they have to put up with. By helping them out (sometimes a little bit too much), I gain their trust and they actually get to learn something meaningful from time to time. Never underestimate trust.

This week, it seems, students have been learning about electric potential. They are assigned a lot of "exercises". I refuse to call them problems, because there is nothing problematic about them. Take for example, this problem, for which several students called me over to discuss.

The question just asked students to find the potential at a point in space due to three charges. The students had to do a bit of geometry to find some unknown lengths, but otherwise it was a simple straightforward calculation. Each of the student I met had correctly done the calculation. They had called me over because they had gotten the right answer that v= 0. These students must have been surprised by the answer, because they each called me over to discuss, "Does this make sense?"

OK. Stop. Hold the phones. Students spontaneously calling me over to talk about whether an answers makes sense. NOT, "do I have right answer?" NOT, "how do I do this?" They wanted to talk about whether or not an answer made sense.

And this is where I had to sigh. Because saying that the v =0 doesn't mean much. Really, you have to know what the potential is at nearby points to say anything. Because knowing differences in potential tells you something about electric fields and/or where charges are likely to move. Sure, sure, maybe you can make some argument about how it tells you that the potential there is the same as the potential at infinity. And then we can talk about how much net work it would take to bring in a particular from infinity. Sure, sure, sure. But what does v =0 mean? Not much.

So, here these students are, and they have been asked to do this "exercise" by performing some rote calculation using the formula k*q/r. They do it well. And they are puzzled? And I have to be the one to tell them that answer is meaningless, pretty much anyway. Not that their answers is meaningless, but that any answers would be meaningless. (Note: We did talk about why it was meaningless; and what else you would need to know for it be meaningful. But I digress)

Here's why I am so frustrated. We complain so much about students not stopping at the end of the problem to ask, 'Does this make sense?' But I'm the one who has to look them in the face and say, well, your professor has assigned you a problem that isn't about making sense of anything. He just wanted you to do some push-ups. I have to say that the best we can do is to check your work and make sure you calculated it correctly, and maybe to offer a mathematical explanation for why it seems plausible (based on geometry) that it could work out to zero. But I can't offer them an answer to the question, "Does it make sense?" from a physical sense without really twisting things around and bringing in a lot of baggage about potential energy, work, infinity, etc.

Am I wrong? Can I make meaning out of this stupid calculation students have been asked to do?


  1. Funny, I just wrote a post on the internet about some problems I found on the internet aren't realistic and don't make sense. This must be an epidemic or something.

    I think it's difficult for one step calculation problems to really push students to think about the significance of their answer and whether it is realistic

  2. It's hard to say that the problem is "good" or "bad' without knowing a lot more about the course or the prof. I have taught college courses for many a year now and sometimes, students need to do pushups before they can do anything else. And maybe, just maybe, the prof wanted the students to look around the room at the end of the problem and wonder, "does this make sense?" Or maybe, the problem got assigned by accident five years ago and it's never been changed.

    Guilty, on one or two counts.

  3. The first year I taught intro physics I looked at the exercises at the back of the chapter and decided to only assign the synthesis problems and not the "push up" ones. The students didn't take to it well. First, they were used to getting some free/easy points for easy problems. Second, they said they were struggling with easy things. I asked if they were doing the easy ones anyways and, of course, they said no. They wanted me to assign them so they could get credit for the easy ones. That semester I received arguably my worst student evals ever. I've struggled ever since to try to get my students to just do the push ups on their own. I don't often succeed.

  4. Thanks for the comments everyone. It is part of my therapy at the end of a long Thursday.

    @Rick. So, I'm not trying not to say it's a good or bad problem, because I don't think it's really a problem at all. It was an exercise, and I don't think exercises or "push ups" are bad, necessarily. We all need practice.

    I just think we need to let students know the difference. When you play sports, you know the difference between "running drills" and "practicing the game" and "playing a real match"

    Because we can't have it both ways when exercises and problems look and feel the same. We can't blame students for not asking, "Does this make sense?"when 90% of the stuff we make them do can't be made sense of.

    And yes, I have sympathy for professors. Professors are also victims of this broken system.

    @ Andy. I agree that students need practice. But practice needs to be focused. For homework, I'd rather assign ten exercises where students just have to draw a freebody diagram given a situation (and nothing else) than ten exercises that practice 12 different skills. I think my evals go up with I give students focused practice over fragmented practice.