# Day 132: COM and Developing Eelas

NOTE: This post is being written one day late… I watched the NCAA tourney game last night, so this post was preempted.

We started the class by discussing any questions the students had about some 1D conservation of momentum problems.  There were a few, but not many…. meaning the problems were too easy or they did not do them.  As it turns out, it was half and half; those that did the problems found them pretty easy, those that had not bothered really did not know where to start.  As part of the solution each problem required and IF Chart.  If (not pun intended) you do not know what IF charts are, take a look at this post from Kelly O’Shea.  The charts re pretty awesome.  My experience has been that most students do not have any trouble solving 1D COM problems… especially since the equation is always the same.  TO me, the real power if the IF chart is in answering conceptual questions… like after two objects collide inelastically, which way and how fast…  It was here that there were more questions.

Anyway, after a few minutes of questions I put the students in to groups of three work on a single 2D collision and an inelastic collision that combines with energy concepts to answer a question.  I let them work for about 30 minutes. With about 10 or 15 minutes left, I introduced the activity that we would work on tomorrow, but that they needed to get ready for.  It is the ‘Physics Face-Off’, developed by Peter Bohacek.  Here is a link to it.  In short , it uses Direct Measurement Video’s to have student groups create a challenging problem for each other.  More in the next post.

General Physics:

Today we graphically developed the model for elastic energy from the area under a Hooke’s Law graph (force applied vs. change in length).  We had already established that the area trapped was the elastic energy, today the students plotted the elastic energy as a function of change in length graph.  To make it go a bit faster and to conceptualize it for the students, we had the start with Logger Pro made Hooke’s Law graph.  From here, they choose a change in length off the graph,  used the examine feature to find the corresponding force, the calculate the area of the triangle (elastic energy).  Al this was explained in the Logger Pro file.  We simply added a text box with the basic instructions: Following the steps creates the non-linear elastic energy vs. change in length graph: Linearizing it: Notice how the slope of this linear graph has the same units as the spring constant, but the value is 1/2k… and there is it is…. in linear form:    Eelas =1/2kΔx^2

Some may ask, why not just give the equation to the students or derive it for them??  Then it is me doing the work and thinking, not my students.  Nuff said!