"Group Quiz" on impulse-momentum

Happy and Sad Balls -- which one produces
more force when dropped onto a force plate?

I can't count the number of articles I read that sanctimoniously preach how physics teachers need to "actively engage learners," involve students in "peer instruction,", provide "inquiry-based interactions", or any other set of edu-buzzwords you can create.  These articles push a fundamentally correct point: that I'll have enormously less success if I merely talk at the white board than if I somehow get the class to involve themselves in the topic at hand.

But as with any other educational method, active engagement only works if it's done right.  The trick is to get students to care about the answer to the question you posed, and about the justification of that answer.  I don't want to read any other literature telling me that active engagement can be effective.  I want to know specifically how other successful physics teachers get their students to actively engage.

I incessantly ask "check your neighbor" questions, in which I give students time to write an answer; I give time for class discussion; and then I survey the class, or call on a random student to summarize his thoughts.  These are generally effective.  However, after a few weeks, the shine has gone off of this novel (to the students) activity.  I can see the beginnings of apathy cross my students' faces... "Oh, again with the neighbor arguing thing.  Gee whiz."

I've got to vary my approach if I'm going to keep class activities fresh and interesting.  I tend to ratchet up the reward for correctly justified answers to my check-your-neighbor questions.  One thought that I've detailed previously is to call on a random student after discussion... if that student can clearly and correctly answer my question, I'll cancel the next day's quiz. 

I generally give a daily quiz at the beginning of class.*  My colleague Paul Vickers modified my daily quiz to an occasional "group quiz," in which he assigned groups of 2-8 students to answer a check-your-neighbor-style question for a quiz grade.  The fact that it's called a "quiz," that the students perceive that their performance will directly affect their grade, keeps everyone focused and on-task.  Yesterday, I tried a new hybrid approach to a check-your-neighbor question.

* Why?  Because students *care* whether they get the answers right, so they pay attention when I go over the quiz better than they would pay attention to the same conversation without the context of a quiz.

The question:  I have a happy ball (one that bounces nearly to the height from which it was dropped) and a sad ball (one that hardly bounces at all).  I drop each ball from the same height onto a force plate.  Both balls have the same mass; both balls are in contact with the scale for approximately the same time.  

Question 1:  Which ball experiences a bigger momentum change?

Question 2:  Which ball causes a larger reading on the force plate?

The method:  I began like a standard check-your-neighbor question.  I wrote the questions on the board, and asked the students to write and justify an answer in their notebook.  After about a minute or two, I asked everyone to argue with his neighbor.  Nothing to see here, really; I did let the discussion go on a bit longer than usual, making sure that those who were still making physics points to each other had a chance to hash out any disagreements.  

Finally, I gave everyone a blank card.  I told them to write and justify the answer to each question as if it were a quiz.  I promised that I would choose a student's card at random to read to the class.  A correct answer with justification on the card would be worth an extra credit point for EVERYONE on that day's quiz.

Oh, boy, did I get careful justifications.  One class's random delegate explained the answer perfectly, earning the credit with no doubt.  The other class's delegate explained beautifully (but incorrectly) that since the balls have the same weight, the force plate must read the same value, and thus both balls will have the same momentum change.  Knowing that many class members had convinced themselves of this mistaken fact, we talked about why the force plate would NOT read the weight of the ball.  

Right or wrong, making the check-your-neighbor question into a quasi-quiz convinced all my students to write clear descriptions of their thoughts.  Even though I only looked at one answer per class, everyone took the writing seriously, and everyone could evaluate for himself the quality of his arguments.  

I may get away with this quasi-group-quiz once or twice more before it becomes just another day of class.  Then I'll have to provide a different sort of incentive for careful, invested participation.  I'm open to ideas -- email me, or post a comment.

GCJ