# Day 129: Conservation of Momentum WB and Hooke’s Law

Today we finally were able to have the post-lab discussion on the collisions experiment.  It was a bit of a different style of discussion. I had each group prep a WB with a description of each collision it studied, they demonstrated each one and we viewed the movie the group posted.  I also had the group provide the initial and final momenta values for each collision, then we looked at the pooled data from all of the classes for that type of collision.  For the most part our results were pretty decent.  We had wanted a slope of 1.0 to establish the conservation of momentum.  The inelastic collisions were the most accurate. I think this is because there is only 1 ‘thing’ (the connected) carts to determine the velocity for.  Next year I think I need re-enforce the concept that we really need the instantaneous velocity right where the carts collide.  I had a few groups put on a linear fit so the velocity was a bit off.  Tomorrow we will compare the total initial kinetic energy and total final kinetic energy.  As the homework for tonight, each group needed to simply add a column or three to the spreadsheet so the kinetic energies could be calculated.

General Physics:

Today we had the WB discussion on the Hooke;’s Law experiment with the springs.  For some reason, this time around, quite a few groups had significant intercepts… I even had a few groups re-gather the data with the same results.  Interesting enough, the slope values were pretty much spot on with the given values.  Tomorrow, we are planning to practice with the spring constant concept by asking a few ranking task and quantitative questions.

# Day 128: Back At It

Spring Break in Neenah has come and gone… now it’s the final push to the end of the year.  I always get mixed feelings after spring break.  Panic… sooo much yet to cover (well to learn); Pride– all the students (well most, … no all) have grown so much and are much better (physics) students than they realize; Sadness… this group of students will be moving on, that’s sad (for the most part); Frustration… I still have not figured out how to reach a few students. No matter what I try, it just does not seem to work.

Today was to be the day we had the post – lab discussion for the collisions experiment we started on the Thursday before the spring break.  It seems to be taking longer this year.  I did a few new things this time around.  Each group shot a quick movie of collisions they studied.  Here is an example:

And one more.

I also required the students to use a spreadsheet (or Logger pro with calculated columns) to complete all the calculations. All of the data will be pooled and dumped into one Logger pro file to create a graph of total initial momentum vs. total final momentum.  If the slope turns out to be equal to 1.0, then we know that momentum is conserved.  The new part for me was setting up a google spreadsheet for the (CAPP) students that were required to complete the analysis of a 2D collision using our hover pucks or our air hockey table.  I’m a bit ashamed to admit it, but this is the first time I have used a google spreadsheet with my classes.  It is awesome to sit and watch the data get entered.  Hopefully we will be able to see momentum conserved in both the x and the y directions.  I will probably not go into any detail about the center of mass of a system aspect unless the opportunity presents itself.  In my experience, conserving momentum in two separate directions.

SIDEBAR:  In my mind I am wrestling with my sequence.  I have been teaching momentum( and impulse) AFTER Energy and dynamics.  As of late, though I have read more about teaching momentum (and impulse) BEFORE energy and even before some aspect of dynamics.  I’m just not sure if there is a pedagogical advantage one way or the other.

General Physics:

Today we discussed the Energy bar chart (LOL’s) assessment they took the day before we went on break.  We also had the pre-lab discussion for the Hooke’s Law experiment.  I do not do the traditional Hooke’s Law Experiment with masses being hung on a vertical spring.  I phrase the purpose to include something like this:….  determine the relationship between the force exerted on a spring and the change in length of the spring.  I find it makes it easier for the students to transition to a compression spring because the ‘change in length’ could be a ‘get longer change in length’ (like our extension springs) or a ‘get shorter change in length (like a compression spring that will be used with the Pasco cart launchers). Here is a picture of how the data is gathered:

The spring are pulled horizontally so there is no confusion about any gravitational affects.  This also makes it much easier to transition to the compression spring and determine it’s spring constant.  Each group did two springs to see that the slope of the linear graph does depend on the spring AND to have its data confirmed.