Built on Facts

Testing 1,2,3

A question before the physics: I hear Hillary Clinton is being considered for a position as Secretary of State. Let’s say this is true. Why would a senator want to take that job? It’s a temporary position. Eight years max, not much longer than a single term in the senate. Four years if the president doesn’t get re-elected. In the senate you set your own views and political objectives. In a cabinet position you work under the orders and at the pleasure of the president. If you disagree with the president, too bad for you. Both your ideals and your reputation are largely at the mercy of the policy he chooses to set. And once it’s done you don’t have your old senate job waiting for you. A new incumbent will have been sitting in your seat for years and incumbent senators are notoriously hard to dislodge.

So I could be wrong, but it seems like the cabinet is a step down from the senate. Why would anyone want it?

Anyway: physics. Now as this week is the last week of my teaching Physics 201 this semester, I think it’s appropriate to quote this post from blogger Rachel Lucas, who is taking a chemistry class this semester at her university. She’s 36, and does not have a lot of patience for ignorance. I don’t blame her.

Lab partner: “Well now we must make the solutions stronger to test higher concentration.”

Me: “We can’t make them stronger, they’re from stock solution.”

LP: “So we just add more!”

Me: “That won’t make the concentration go up.”

LP: “Yes it will, we just add more!”

Me: “THAT WON’T INCREASE THE CONCENTRATION.”

LP: “Yes it will! If you add more to what is here, there will BE MORE.”

Me: “More volume! Not higher concentration!”

Ah Rachel, I know how you feel. Physics 201 labs are blisteringly easy so there’s not so much that I see that’s as bad as that. Still, sometimes you get questions which test your faith in humanity. I have a lot of patience though.

In fact, when grading exams with other TAs, sometimes we see things that we think should almost be worth negative points. Take this particular mathematical manipulation, which I’ve seen more than once. Usually on tests that end up with very low numbers written on the front. They take the left side, “cancel” the m, and get the thing on the right:

i-d4bb9fe09d6b78eb832985821d8a307f-1.png

Blarg.

I can’t complain though. Most of my students are very good, and I wish them all the best on the upcoming final. Having stuck it out this long and being the ones who didn’t drop, they’ve done a pretty good job.

As a veteran of many a physics test, I can say that there is absolutely nothing that will help you more than getting out your books and doing the problems until you drop. Then do it again the next day. Lather, rinse, repeat until exam day. If you don’t know how to do a problem, keep working on it until you figure it out. If you can’t, ask someone. Friends, TAs, and professors (in that order!). I don’t think any of my students read this, but it’s good advice for anyone in a physics class. Good luck!

Comments

  1. #1 dean
    November 18, 2008

    “there is absolutely nothing that will help you more than getting out your books and doing the problems until you drop. Then do it again the next day. Lather, rinse, repeat until exam day. If you don’t know how to do a problem, keep working on it until you figure it out. If you can’t, ask someone. Friends, TAs, and professors (in that order!). I don’t think any of my students read this, but it’s good advice for anyone in a physics class. Good luck!”

    I teach mathematics and statistics: those subjects could replace the word “physics” and the result would still be excellent advice.

  2. #2 Bob
    November 18, 2008

    A colleague of mine, now unfortunately dead, maintained that a student had to do a particular kind of problem 30 to 40 times before he mastered it.

  3. #3 dkw
    November 18, 2008

    “So I could be wrong, but it seems like the cabinet is a step down from the senate. Why would anyone want it?”

    Two reasons: (1) People don’t like Congress. Congress routinely has lower approval ratings than the executive branch because it always seems like Congress is at a stalemate and nothing gets done.

    (2) Secretary of State is a way to heap on foreign policiy experience.

    This brings me to the only “real” reason she would do it: To position herself for another presidential run in 2016.

    Of course there is the off chance that she’s winding down her political career and being SoS just means that she’ll never need the stresses of campaigning and fundraising again, but she’ll only be 69 if elected in 2016, and that’s not too old to run again.

  4. #4 Blaise Pascal
    November 18, 2008

    A Senator may have a particular field of interest, but really can’t concentrate on it as much as he/she would like. There’s lots of things needing Senatorial attention and limited time to do it. There’s also somewhat limited influence, as there are 99 other Senators who get an opportunity for a say in everything. Even then, the power of the Senate to set policy is limited.

    In contrast, a Cabinet Secretary’s day job is to set policy and manage one particular policy field. They are involved in how the day-to-day implementation of the policies set by him/herself, the President, and Congress gets carried out. If a Senator’s main interest is in that field, he/she may feel he/she can get more done as a Secretary than as a Senator.

    Also, for a Senator with Presidential ambitions, but is seen as lacking in “executive experience”, a Cabinet Secretary position can be a good resume booster. And Secretary of State is a good position for handling international affairs.

    Can’t you put the solution under an vacuum evaporator to increase the concentration?

    And how about (F-mg)/m –> V-g? That potentially sounds viable.

  5. #5 Bob
    November 18, 2008

    Blaise,

    I’m not seeing (F-mg)/m –> V-g. Now (F-mg)/m –> a – g I can buy, assuming that we’re dealing with F=ma, which makes sense on a physics blog.

  6. #6 Blaise Pascal
    November 18, 2008

    Bob:

    You’re right; I was thinking of “force per unit something” as potential, but m is appropriate as the something only for gravitation, in which case you get g as the potential anyway.

    For a non-gravitational force F, F/m isn’t a meaningful potential, so V, which seems common for a potential, isn’t reasonable.

    And I think I’m not using the right definition for potential (or, using the wrong word for “force/unit something”). I think I’m talking about field strength, perhaps.

  7. #7 Chris
    November 18, 2008

    I definitely don’t “know politics” very well, but I think that Hilary has a fairly weak position in the Senate, and people will be watching for her to break with Obama on any issue. So, in a sense, she’d be hamstrung there. If she takes SoS, she’s basically expected to “work under the orders of the president” which sounds like a load off of the shoulders, to me.

  8. #8 D. C. Sessions
    November 18, 2008

    As a veteran of many a physics test, I can say that there is absolutely nothing that will help you more than getting out your books and doing the problems until you drop.

    A subtle quibble: playing with the material is somewhat more effective than working at it, since play is self-rewarding and as a result “sticks” better. When you get to the point where you see the physics in question everywhere and start doing the analysis, you’re well on your way there.

    Of course, play is not always an achievable state of mind, and it’s necessary to be able to work at problems even when they’re not play.

  9. #9 Joshua Zelinsky
    November 18, 2008

    Re: m canceling. Most of the time I’ve seen students make mistakes like that when they are pointed out to them the students immediately realize what is wrong. Part of the issue seems to be that people often do bad algebra when under stress. People who do particularly badly under stress are more likely to do this sort of thing. The fraction of students who are genuinely that bad at algebra to do that under calm circumstances is small.

    The lab assistant is just an idiot. There’s no excuse there.

  10. #10 Eric Lund
    November 18, 2008

    Blarg indeed. That looks like the same kind of idea as the “proof” I saw as an undergrad that (sin x)/n = 6: Cancel the n’s, and you’re left with six. But we understood that that was a joke. I’m lucky not to have seen many such examples where it was meant seriously.

    Then again, sometimes these idiots survive through graduate school. Case in point: I once reviewed (and recommended rejection of) a manuscript which used a non-dimensionless parameter as the argument of a non-polynomial function. As most of us learned in grad school if not earlier, such arguments must be dimensionless.

  11. #11 Uncle Al
    November 18, 2008

    Regarding [Hillary]: A Senator is corrupt on a national scale. A Secretary of State is a planetary whore. Consider all the good works Henry Kissinger has engaged stroking the powerful and poking the powerless.

    4 – 10 = 9 – 15
    Add 25/4 to both sides,
    4 – 10 + 25/4 = 9 – 15 + 25/4
    Write sides as complete squares,
    (2 – 5/2)^2 = (3 – 5/2)^2
    Take the square root of both sides
    2 – 5/2 = 3 – 5/2,
    add 5/2 to both sides
    2=3

    We all know
    (pi)^4 + (pi)^5 = e^6
    31, 331, 3331… are primes
    n^2 + n + 41 generates primes for n = 1,2,3…

    “Correct answers” are discriminatory. Inert intelligence is the paradigm of institutional racism.

    Editor’s note: disparaging puns based on female anatomy are not going to fly here. Please try to stay tasteful, and this goes for all commenters.
    - Matt.

  12. #12 andy.s
    November 18, 2008

    Election year rhetoric notwithstanding, there was very little difference in policy between Hillary and Obama. I suspect they would see eye-to-eye on a lot of issues.

    Maybe she’s in it for the pay raise: $191,300 vs $162,100.

    So do you have any advice for people who study on their own in physics or other masochists?

  13. #13 Bob
    November 18, 2008

    andy.s

    I’ve studied physics on my own from time to time and I think the same advice holds for us autodidacts. Grab a physics text and do the problems. Reading the text gives you the illusion of understanding (it all looks easy when someone else does it!). Doing the problems leads to genuine understanding.

  14. #14 llewelly
    November 18, 2008

    In a cabinet position you work under the orders and at the pleasure of the president. If you disagree with the president, too bad for you.

    Usually, but not always. Lincoln, FDR, Eisenhower, and Kennedy (after he learned his lesson at the Bay of Pigs) all retained several cabinet members who disagreed with – and had their viewpoints and policies changed by cabinet members they disagreed with. It’s been argued that listening to and retaining cabinet members they disagreed with enabled these presidents to make superior policy.

  15. #15 Anonymous
    November 18, 2008

    I had the same argument in a chemistry lab the other day. The only reason i have done well in my physics class’s is the problem after problem approach it has worked so far same with the C++ and chem and bio and maths. Just keep banging your head against it and you will understand the problems pretty quickly. English on the other hand…..

  16. #16 CCPhysicist
    November 19, 2008

    #9, you need to read Rachel’s blog. Her lab partner (not the lab’s TA) making those “Proof by emphatic assertion” observations during lab claims to be a senior engineering major. The bit about increasing the drip rate in a filtering experiment by raising the filter is a classic.

    And Matt, that is not a high-level class. It is first semester chemistry, presumably already passed by your physics students.

    As for canceling m, this sort of error (failure to distribute division) is actually quite common at the level of math we teach before college algebra at the CC. It is probably second only to failure to distribute subtraction, as in such abominations as -(x-3) = -x-3. Students who got through algebra and trig using a calculator to solve problems could still have serious latent algebra weaknesses that show up in the other numbers on the front of the exam.

    But how did they get through calculus? Or does your class have calculus as a co-requisite?

  17. #17 gah
    November 19, 2008

    My all time favorite is, taking the limit as x->0, (sin x)/x –> sin

  18. #18 Matt Springer
    November 19, 2008

    This is the non-calc-based intro physics for majors other than the physical sciences and engineering. It’s the “easiest” of the physics classes we offer.

    I think the greatest number of the students are med school hopefuls, which is sometimes slightly alarming.

  19. #19 andy.s
    November 19, 2008

    Grab a physics text and do the problems. Reading the text gives you the illusion of understanding (it all looks easy when someone else does it!). Doing the problems leads to genuine understanding.

    I do that, but it takes a while. Now if only Matt would volunteer to grade me :)

  20. #20 Peter
    November 19, 2008

    I agree in part with Joshua- student are thinking about if they are doing the physics right and forget how to do the math. I made it a point to separate the two – which is possible since all the physics is in the setup that leads to some equations and from that point it is just math. Same with any of the advanced maths – it is too easy to make a arithmetic mistake if you are still thinking about some calculus – set up the problem then just do the arithmetic without thinking about the math. Alternate doing math or arithmetic but don’t try and do both at the same time.
    Pre-med students with great marks in highschool by good memories and ability to plug numbers into the correct equation often falter in physics when required to make the transition to understanding of a technique (such as free body diagrams) that allows a basic concept to apply to such a broad range of problems they can not possibly memorize every one that could show up on the test.

  21. #21 Bill
    November 19, 2008

    While you college-level physics teachers are “blarging” about some less than insightful exam responses that you have seen in your classes, let me offer a noteworthy response that I received from an 8th grade physics student a few years ago(Yes, there are a few schools teaching physics at the middle school level). The question was,”What is the result of cutting a bar magnet in half? Explain why.” (I was looking for a discussion of magnetic domains)The explanation offered by one of my students was,”You get two new magnets, because its like trying to cut the left end off a broomstick; you just can’t do it.” I gave that answer full credit.

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