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19 February 2016

Raise the stakes -- Get students to care deeply about their prediction

The scourge of the physics student is the tendency to get problems done merely for the sake of finishing the assignment, without really knowing or caring whether the approach is correct.  I'm asked all the time "Can I come to consultation period so you can give me more practice problems to do?"  I generally have to explain that it's not doing MORE practice problems that will help their understanding, it's paying careful attention to the practice problems that I already assign.

A couple of weeks ago I assigned this classic question about dragging a block at constant speed across a table: when I double the speed, what happens to the force I'm pulling with?  Most everyone generally gets this wrong on their first, individual attempt.  They revert to "logic" and "of course" in their justifications.  Even students who have carefully trained themselves to write facts of physics and connect those facts logically to the situation get the answer wrong -- they write a fact, yes, but then say "logically, of course, doubling the speed means doubling the force."  Or they write Newton's second law, TELL ME CLEARLY that acceleration is zero in both cases, then say "since the a doubled, the force must double as well."  

I'm not stating a new issue here -- all the physics teachers reading this are nodding their heads.  We will never get all of our students to answer this question the right way.   There's no magic, secret method that will turn all students into Newton's second law machines.   All we can do is use all of our tricks to maximize the number who do it right. 

Underlying all those tricks, though, is getting students to care very deeply about their prediction.  I often pose the thought experiment:  Will you bet $100 that your answer is right?  So often, the answer is, "sure, but let me change my answer real quick."  I then have the conversation about how they hadn't taken their initial answer seriously enough.

But that's a thought experiment.  Everyone knows I'm not REALLY going to bet them $100.  If nothing else, they know I'd always win.*  My point stands: there's no purpose in doing a physics problem if you don't really believe in your answer.  If students had to gamble money on the correctness of each answer, their problem sets would be vastly improved.

* I do get the occasional student willing to bet, reasoning that while I'd certainly win, they'd get their money back after I was sacked for gambling.

Given the prohibition against actually insisting our students bet money, can we instead set up a situation in which the students FEEL like there are high stakes to their prediction?

Some folks would put a grade on the line -- "this correct answer earns extra credit."  Sure.  But to me, that feels no different than grading their problem set.  If 4/5 of the students are already getting the answer wrong on a graded problem set, then clearly grades are an insufficient motivator.  The stakes have to be different.  Money isn't an option... but you have other "items" of value that you can use.

Last week I used gift certificates to the school snack bar.  Each certificate was for a single item -- a cheeseburger, order of fries, milkshake, whatever.  I've also used homework exemptions in the past; or, the ability to leave class early rather than work on a review packet.  These are all high-stakes items to my students.  They covet, ninth commandment be danged.  (Which is silly, when you think about it.  They're all paying some fraction of the astronomical school tuition to attend here, but they go nuts over a cheeseburger worth $1.65 at our subsidized snack bar.)

Now, here's the new bit -- I didn't just say "correct predictions earn a gift certificate."  No.  That wouldn't give the students any skin in the game.  There's not much given up if the prediction is wrong -- oh, well, no cheeseburger for me.

Instead I gave each student a gift certificate.  Everyone held, in his very hand, a piece of paper worth the coveted cheeseburger, with his name on it, even.  

Then, each student had to place his certificate on top of a card indicating one of the possible predictions: the force necessary to move the car will double, quadruple, or stay the same.  Only the certificates placed in the correct pile would be returned.

I've never seen such investment in the results.  You've heard the term "bear pit atmosphere?"  I've never been in a bear pit, but I've seen the reactions around a high-roller craps table.  Suddenly, my class was all nervous, excited, anticipatory... of the reading on a force probe.  The difference was, with the certificate in-hand, they were worried they might lose what they already felt like they had.  Why should there be a difference between "correct predictions earn a cheeseburger" and "here's a cheeseburger, now you can only keep it if you make a correct prediction?"  I know I've seen Nate Silver and his ilk discuss the psychological research.  I make no claims to know why this technique felt different, I only know that it worked. 

When, later that week, I had students write a correction on that problem, I got immediate, thorough, and correct justifications.  No more of this random wrong answer for the second and third time with a hangdog look telling me how hard physics is... this time, they remembered the answer and the justification -- because they "lost" a cheeseburger.  :-)


  1. Greg, Your story is quite funny. I may have to try this. My students tend to be older and don't need much prodding. Today we examined the free response question from last year's AP Physics 2 exam. It was the optics question and within minutes at least half of them insisted that the answer to part b I I was incorrect. I was sure they were right but the AP answer says we are wrong. Have you checked it out? I have been told that the AP graders did not take alternative answers.

  2. Rick, the solution to 2015 P2 #1-b-ii I find on AP central is correct, as far as I (and the table leaders and the readers) can tell. What is your alternative explanation? I mean, you could phrase it as an amplitude increase, which is the same as an energy increase, but it is correct that the brightness increases because previously the light had both refracted and reflected off of the air interface, and now all the light reflects there, leaving more energy available for the dot at point Y.

  3. Today we set it up for real and observed. Then we set it up using the pHET simulation and...we agree with AP. What I loved was that my students questioned the answer as given. In a way they were right ; the x point does get slightly brighter (1.5%) but the Y dot gets over 20% brighter. AP wins this one :)

    Thanks for your response.

  4. Ooh... I'd love to see a picture and results from a live setup, where you measure the brightness of the laser with a Vernier brightness probe. Awesome! Great idea, setting that up on the simulation.

  5. Hi,
    I have been reading this blog for a long while now and this is the first time I comment. I have got so many great ideas from here that I feel I need to give something back!
    Your post got me thinking: how do I get students to care more about their answers? As you said, there has to be something at stake. However, we don't have a cafeteria here at school and I would like to make my rewards more physics-based ("physics is its own reward" and all that). So here's what I came up with:
    I made my own "physics" currency (the "quark"). Then I gave each student a cup with their name and 5 quarks. I then made a list of possible things they can buy with those quarks: they go from the silly (playing with the hoverball during a break) to "plasma ball demonstration" to "exempt from one random quiz." I also randomize seats, so one of the options is to "buy" their seat for one day.
    Then I got them to bet on their answers any time I wanted them to really think things through. I forced a minimum bet of 1 quark, but they could bet as much as they wanted. I've never seen them so engaged! They suddenly "needed more time to think" as opposed to being done in 20 seconds.
    I also made it so that, when they run out of quarks, they can sell me things in exchange for them. For example, they can sell me extra notes, "concept tables" (a revision tool I make the use) or attempted difficult problems from previous topics.
    This has created a whole new dynamics in my class.
    I have also adapted one of your ideas from a previous post (the 5-minute quiz at the start). I do the same, but my quizzes are exactly one question and its topic is random (I generate it randomly from an Excel spreadsheet I made). This means that they need to constantly revise previous topics if they want to do well. The questions are normally conceptual and based on common misconceptions (such as the centripetal force pointing out).
    Anyway, I find these have really helped my class and it's all thanks to your great ideas. If you want any further details on the implementation, don't hesitate to ask!

  6. Eduardo, that a fantastic idea, worthy of exposure in a full post soon. I know that Staci Murray uses stickers -- yes, stickers, the things we all collected in a book when we were in third grade -- in a similar way. I use skee-ball tickets and extensions as well as cheeseburgers. Anyone else use alternative currency in your class?