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04 July 2015

AP Physics 1 lab ideas: Spring constant of a hopping spring toy

spring toy available
from Oriental Trading
For the first time in ten months I'm not in constant preparation mode -- preparation for class, for workshops, for the USIYPT, for department meetings... I have a few weeks off with no immediate obligations.  Now is a good time to take stock of the activities I used in my first year of teaching AP Physics 1.

My advice about laboratory in AP Physics 1 is to teach the material up front and quickly, such that students know and can use basic facts, equations, and problem solving techniques.  Then do experimental work.  Set up problems you've solved for homework or on quizzes as laboratory activities.  Some of these will be quick and dirty -- did the speed at the bottom of the hill double when the hill's height quadrupled?  Some will be more involved, with extensive data collection and graphical analysis.  

Over the next few posts, I'll describe some of the extensive, long-form laboratory activities I used this year that were successful.  Many of these are based on old AP Physics B questions.  I'd suggest that scouring the released AP Physics B free response questions since 1996 could provide a fantastic lab manual for any advanced physics course.  

Today's experiment comes directly from the 2009 AP Physics B exam problem 1.  I show the class how the pictured pop-up spring toy works: push it down, and when the suction cup loses suckiness, the toy pops up.  I show them that we can use flexible aluminum wire wrapped around the top to change the toy's mass while still allowing it to pop up.  I ask them to graph the height to which the toy pops as a function of the toy's mass.  As always, I give no handout with instructions or prompts.  Each group is to produce and turn in a raw graph of the experimental data along with a data table.

When I'm satisfied with the data collection, I xerox the data tables so that each student in the group has his own copy.  Then I hand out the linked homework assignment.  Each individual student now must linearize the graph, take the slope, and use the slope to determine the spring constant of the toy.  
Before you try this experiment, be sure you've done at least two or three graph linearization experiments, and be sure everyone can deal with basic energy conversion problems.  I never did assign the official 2009 AP Physics B problem.  However, if you use that problem on a test, then this experiment could be a perfect follow-up.  Or, do the experiment in November, and put the 2009 problem on the semester exam in late January.*

* That's not as crazy as it sounds.  Students don't remember your lab exercises as much as you think.  For example, my class did the "does a light bulb obey ohm's law?" experiment this year, with everyone making a graph of voltage vs. current to see if the slope was constant.  Nevertheless, when we debriefed and discussed the 2015 AP Physics 1 free response, no one at all mentioned that we had done that very experiment.  Most of my students got it essentially right, but without the elegant graphical solution -- they said they checked several times to see if the V/I ratio was constant.  That's Rule 3 of teaching: Your students don't listen to you.  (But no worries, they don't listen to me, either.)

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