# Day 110: Hooke’s Law Research and Developing Elastic Energy

Over the last 3.5 days of class (yes, another 1/2 in there), the students completed and independent research experiment related to Hooke’s Law.  Essentially they came up with one change they wanted to make to a spring/spring system, made a hypothesis about how the change would affect the spring constant, then gathered and analyzed the data to verify the hypothesis.  They also posted their results on a shared Schoology discussion and made at least three comments regarding other experiments.  Now, truth be told, some of the experiments reached conclusions that may not be accurate, but I’m OK with this because the focus was on the freedom to just explore and be creative, not necessarily scientifically accurate.  In short, I think is was awesome. I had 2 groups compare the ‘k’ value in air and in water… yep, into our pool to gather data.  Groups look at temperature… room temperature vs. -17F (~-30F with the windchill).  I need to allow more of this.. just need to find the time.

Today we connected Hooke’s law back to energy.  We discussed how when we exert a force to cause a change in length of a spring, we are also storing elastic energy in the spring. We plotted force as a function of change in length (the traditional Hooke’s Law graph), but did not  have any energy into the graph yet.  We discussed where the energy might be hiding.  After looking at the energy bars for several changes in length and the Hooke’s Law graph, we defined the area trapped as the (working) elastic energy.

The next step was to plot a graph of elastic energy as a function of change in length.  To determine the energy values, we used the integral feature of Logger Pro to find the energy at 8 changes in length. We did not use our original Hooke’s Law graph, we used a new one plotted from the Hooke’s Law EOL and generated change in length values.  We did this to ‘smooth’ the data and to provide more to work with.  Tomorrow is the post ‘lab’ discussion on this.

General Physics:

Tomorrow we will have the post lab discussion for the Modified Atwood machine experiment that develops Newton’s 2nd Law.It is essentially the same experiment the advanced students completed, but the general students only complete one part, either acceleration as a function of mass (with a constant net force) or acceleration as a function of net force (with a constant mass system).

# Day 80: N2L Data Gathering Set-up

As I explained yesterday, we are completing the data gathering for the Newton’s 2nd Law experiment using a modified Atwood Machine.  Here is a screen shot of some of the data gathered, and a movie showing the actual process.

As you can see, the process gives pretty nice data that is easy for the students to analyze, more importantly, understand where the acceleration is being gathered from.

General Physics:

Today was the WB discussion for the Video Analysis of a projectile experiment… nothing really that interesting to show because I talked about it here.

# Day 79: N2L Paradigm Experiment and VA of a Projectile

Well, the first day back after a long break is always tough.  Today was made a bit tougher because most students were thinking we would be under a 2 hour delay because of the weather… -31F with the windchill.  Some of the schools a bit north of us did go 2 hours late and some even cancelled… not use, it was only a Windchill Advisory, not a warning.  I’m actually glad we were not delayed.  We have to get started at so we might as well have a full day.  We actually have only about 7 days left in the semester, better get busy.

Today we had the pre-lab discussion for the N2L paradigm lab.  Like most modelers, I use the modified Atwood experiment, with a twist here and there.  My advanced classed usually do both parts  acceleration vs. mass of the system and acceleration vs. net force.  This year however, to save time, each group only did half of the experiment (odd cards did the first part, evens the second part).

SIDEBAR —> This was the first time in a whole bunch of years, that in one of my classes, net force as a factor affecting the acceleration DID NOT come up, so that class has everyone doing just the mass of the system experiment.

We start the discussion with two systems (the cart on the track, and the hanging mass as the other system).  We do force diagrams for both.  As we draw them, I ask about how the tension force in each diagrams compares.  My classes are pretty good at this point knowing that it is the same in both.  OK, so the Ft is the same in both diagrams, interesting.  I then ask about the acceleration.  It is split about 50-50, some say the acceleration of the phus is equal to the hanging mass, others say the phus is larger.  it only takes a few minutes of discussion for everyone to see that they have the same acceleration.  I have never had to check this with equipment (a motion detector), but next time I may just ask how we could prove it.  So, same tension force and same acceleration, so maybe we could treat the setup as one system rather than two.  This means the tension force becomes part of the system… this is nice because my kids agree it would be very difficult to measure the tension force while the cart is accelerating.

That’s all for now… tomorrow I’ll explain a bit more as the students will finish gathering data.

General Physics:

We started a new unit today with the General kids.  We are taking a look at projectile motion.  The students had a choice.. analyze the Basketball Shot movie that comes with Logger pro as one of the sample movies or shoot their own to analyze.  Either way, tomorrow we will WB the four graphs (x-t, y-t, vx-t, and vy-t).