# 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).