# Day 32: No, the slope is NOT the acceleration and Mirror Mirror

Today we started the WB discussion of the ramp data.  As explained yesterday, I assign the direction and initial position because it makes the post lab discussion more interesting and allow us to address a few things.  Here is a screenshot of the two graphs we ended up with:

So far so good for the students.  There were a few that had to think really hard about what the blue graph was telling us… the (magnitude of the) slope increases in the negative direction so the object is getting faster in the negative direction as it rolls down the ramp.

Next we went to how to make it linear… yes, I do linearization. I find it helpful to work on proportional reasoning , to work with variables only, and to identify what the slope and intercept mean.  So, here is another screenshot of the linear graph:

Ok, we have established the square relationship between time and position.  Now to identify the meaning of the slope and intercept:

We always focus on the units.  The slope units are m/s^2.   I explain that the meters are easy,  they represent a displacement. But the square seconds…. we have no real conceptual basis for that.  What is a square second?  We understand a square meter, it represents an area.  A ‘second’ represents a time, but what about a square second?  By now the math kids are nearly foaming at the mouth.  Finally one of the kids will say, “Yeah, but m/seconds squared is the acceleration, that’s what the slope is”. I simply reply, No, the slope is NOT ‘the acceleration’.  And now some want to argue, some have their confidence shaken and some just want to be told what the hell the slope is (hell is my word, not theirs).

I explain that we are at a dead-end because of this, and when we are in a car, and we hit a dead-end, we go back to where we know something. or us, we go back to our original non-linear position-time graph.  This is where I help the students develop the idea of instantaneous velocity.  Another screen shot:

To help explain the idea of instantaneous velocity, I used the ‘selfie’ analogy (I added Snapchat) I read about on the modeling listserv a while back.  It worked pretty well, especially since I snapped a self in each class that I will start with tomorrow as a review.  I am careful to define instantaneous velocity as shown above and include the position piece.  This is because of where we are headed tomorrow.  Notice not a single mention of acceleration from me.  It will be officially defined tomorrow.  I LOVE THIS STUFF.

General Physics:

After we discussed the test they took yesterday, I shut the lights off and all of a sudden some laser dots floating around the room became visible.  I sprayed some Fog in Can around the round and the beams burst into view.  From a laser on my desk, to a plane mirror on the front board, to a plane mirror up on top of a cabinet, to our disco ball.  GLORIOUS.  So now we are on to mirrors.  Following the same flow as we did with lenses, we start with looking at how the incidence angle compares to the reflected angle.  We use our magnetic laser levels on the main whiteboard and a plane mirror to quickly establish the two angles are equal.  From there we look at how the object distance compares to the image distance and how the object height compares to the image height.  All of this in accomplished by using a CD case and a two identical legos.  I read/heard about the AWESOME idea from Frrank Noschese here.  This is the BEST way I have seen to build these concepts.  GO ahead and watch it, I’ll wait.

See what I mean?  It works so well.  Tomorrow we tackle drawing a ray diagram to show how that image forms.

# Day 31: The Ramp Lab

As mentioned yesterday, today was all about gathering data for the ‘ramp’ lab.  This is the (usual) experiment that develops several concepts, that I’ll discuss tomorrow. I have set this up a number of different ways.  This year I gave the students much more freedom and choice that I have in the past.  They could choose from 3 different objects to send down the ramp.  Their choices were a hot wheels car on its track, a constant acceleration dune buggy (this is the pull back kind, but they free wheel if you do not wind them up), or a large steel marble on a channel made from physics (duck) taping to 10 foot pieces of conduit together.  These are the same pieces of conduit I use a guard rails for our buggy bash.  I like the conduit ramps because it really nicely extend the ramp so the students can get more data, and allows for shallow angles to make the data easier to gate.

In a sense, we repeated the position vs. time experiment we did in the cv unit.

SIDEBAR: When we introduce the lab, it is not one of the ‘does the cv model still work in this situation” intro’s.  I do not do this because I have found the majority of my students already know the cv model will not apply.

I like the similarities because we are still measuring positions and times to describe the motion.  The students were also able to choose a method of timing; stopwatches or photogates (Pulse Timer – Two Photogates).  It totally blows me away that after all we have talked about in terms of our inability to make accurate time measurements by hand, some groups still reach for the stopwatches first.  Maybe it’s because they know them or think it is easier.  The only condition I set is this:  the odds groups (I use playing cards to group my students into lab grops) will put the origin or ‘0’ at the bottom of the ramp and start at some initial positive position, while the evens will put ‘0’ at the top and roll in the positive direction.  I love this when it comes to whiteboarding tomorrow.  It allows me to see who is still stuck on distance travelled instead of position and it awesome when they try to explain the meaning of the slope of the linear graph… stay tuned….

General Physics:

Lens and refraction test today… nothing too exciting.  Tomorrow we start in on reflection and plane mirrors.

# Day 30: Make an answer key, and thin lens practice

If I were a student in class today, I might have been bored.  It was going through the test day.  I have my kids make an answer key because they will keep this test and I want them to have the right answer. I’ll admit though, I hate gong through tests and the time it takes.  If I was sure they would watch it, I might try to do a screencast… maybe I’ll try it anyway.  We also had time to do a bit of pre-lab discussion on the Unit 2 ‘Ramp Lab”, but more on that tomorrow.

General Physics:

Today was all about practicing.  Tomorrow is the first summative test for this group It’ll cover Snell’s Law, Ray diagrams for lenses and Thin Lens problems.  We provided a sheet with four more practice problems for them to work on in small groups.  I know this is good for them because it fosters the study group mentality and I can wander and answers questions, but I don’t like giving up the class time. Two of my colleagues tell me to get over that and my student advisor (my daughter) tells me students LOVE this.  So test tomorrow before we look into mirrors.

# Day 29: A test, this is only a test and Baby Doll Heads (err… I mean an air lens)

It’s Friday and it is the first summative test… loads of nervous physics kids. The summative test was on the constant velocity model.  Nothing too exciting to tell about except maybe the format of the test.  I have a conceptual part (short answer and about 6 MC) and a Problem set (2 problems).  One objective I was assessing was being to solve constant velocity problems.  One thing that I do is offer two levels or problems to assess the same objective.  The first problem is one I thing every single kid should ROCK.  The second is harder and or longer and I expect my top kids to score well on it.  I encourage ALL kids to read  the tougher one and give it a try.  I even offer a few additional points on it because it is more challenging.  No time limit on this test because it is the first one.

SIDEBAR: I always write a different version each year because each year is a bit different and I allow my kids to keep the tests after we go through it.  I think this is an important study tool as they prepare for the final exam… self testing is a researched best practice to prepare for tests so I encourage my students to use it with the old tests. PLUS, each year I have a theme that runs through each test…something that shows up on every test.  One year it was my friend John (because one of his close work colleagues was in my class), some years a video clip… this, well we’ll see.

General Physics:

Today we WB’ed some thin lens quantitative problems.  But we also looked at Baby Heads:

Allowed use to practice and conceptualize when a diverging lens is really a diverging lens and when a converging lens is really a converging lens and virtual images.  Plus, it is really just cool in a weird sort of way.

# Day 28: Reviewing cvpm

Well, not an overly thrilling day in this class, but one the students really need.  Over the last few days, outside of class, the students completed the (modeling) WS (with a few Hertting edits) that puts all four representations together at once; position graphs, velocity graphs, motion maps and a written description.  The only novel thing I add is a roll of a single die after the explanation.  If it comes up as an odd number, the groups models it by walking it for us, if it is an even, then they talk it…. create a story (NOT another description) that models the motion.  In hindsight, I should have maybe played the “Mistake Game” that Kelly O’shea explains here.

First summative test tomorrow!

General Physics:

Today was a bit of a practice day.  We wanted to give the students an extra day to complete the quantitative problems with the thins lens equation and the  similar triangles, so we practice a few of the more interesting ray diagrams.  You know the ones…. give them an image and lens, use a ray diagram to locate where the object had to be….

Tomorrow we WB the problems, always interesting with figuring out the sign convention needed for the virtual image distance and heights.

# Day 27: Buggy Bash Results

Yep, I missed again last night.  But, I have another good reason.  I was invited to speak to a group of students at UWO.  In the College of Education, there is a program called ACT (Alternative Careers in Teaching) for people looking to get into teaching, and get on a fast track to student teaching and licensing.  I was one of five panelists talking about use of technology in the classroom.  I was asked to spend about 30 minutes talking about the use of Logger Pro and how I use our high-speed camera.   Petty easy for me!  It was an interesting evening and I learned about a few new pieces of technology that I did not know about.

We checked the Buggy Bash.   As I mentioned, I like the intersection collision so here are a few clips for you viewing pleasure:

The good:

The really good:

and the ugly:

General Physics:

We did a practice problem involving the thin lens equation.  We set up an optics bench, located an  image and used the object and images distances to determine the focal length. The second type was to use the known focal length to predict where an image would be located with a new object distance.  This one is fun because you have the students predict it and then set the screen, then switch on the light and look for the  perfect image.  It sometimes surprises them when it really is perfectly in focus.  We then talked about how the ray diagram and our linguistic model allows us to predict some information about the size of the image, but the thin lens equation does not.  Enter the need for another (equation) way to determine that… the similar triangles.  Back to practicing, given this object distance and focal length lens, predict the size of the image.  So now we have a two set problem!

# Day 26: An uphill battle, the Buggy Bash and WB’ing the Thin Lens

After discussing the results of the Effectiveness survey the students completed yesterday, we briefly talked about graphical solutions vs. kinematic solutions.  I know this is going to be an uphill battle… helping my students see the sheer power of solving problems graphically.  The overwhelming majority prefer the equation…. just plug and chug… grrr…..   When we get to the constant acceleration problems, we’ll have another go at the graphical vs. kinematic debate. I’m pretty sure with those problems they may see the light.

We also gathered data for the Buggy Bash.   I give each group a buggy, ~20 mins to graphically determine the velocity before I pair them up.  It kills me to see the majority grab watches and tape measures.  I get it, that’s what we did with the buggies last time.  But since then, we have used the motion detectors and video analysis.  Only one or two groups in each class used one of these two approaches.  Believe it or not, I had some graph positions and time by hand on unlined paper and find the slope using the data pints they gathered… killing me I tell you!  It’ll be a rich discussion tomorrow after we’re all done.  I like the intersection crash.  They buggies have to be in motion for at least 7 seconds.  This is just far enough that the two buggy releasers can not see each other because of my lab benches.  Plus it makes it so they can’t just put them don right near each other.

General Physics:

We did a quick in class practice on a ray diagram… predict image characteristics first, then draw the ray diagram to check it. After that we WB’ed the thin lens data.  Some pretty good data:

Look closely, sloped nearly (-1.0) and intercepts almost exactly the inverse of the focal length.  Tomorrow we’ll practice it and develop how to calculate images heights.

# Day 25: And the Teacher Effectiveness Survey Says…… and More optics stuff

Survey day!  As part of the Teacher Effectiveness Project that my district uses for observations, all teachers are required to survey the same group of students twice each year.  I have no issues with this process, I think it is important to know what our students think about our class and about our teaching, we can all improve our practice …. this is a growth mindset for teachers.  Yeah I know they are students, but that does not mean they do not have anything to offer and yes I know, some of the students see it as a joke. (Those responses can be easily identified and eliminated)  Part of that falls to how one presents it and uses it afterward. Yes I said afterward.  I will be sharing some of the results from the survey today… the good, the bad and the ugly.  Even some of the comments.  I think it is all about perspective, talking about it gives me a chance to explain WHY I do certain things in class or explain why I thought I was doing some thing…

SIDEBAR:  I used a Google Form for the survey because I could post the link on Schoology and I really like the way the results can be viewed:

The first 10 or 12 questions or so are required by the district, but then we can add more if we want.

This was the three sections I surveyed.  As I reflected, it may have been too early (even though it must be completed by Oct. 31, this was originally Oct. 15).

After the survey, we continued to work on the graphical and kinematic solutions to the four cv problems.  When each group was finished they ‘purple penned’ it.  This is an awesome method of feedback that I learned about from Frank Noschese here.  (I use purple pens because I could not find orange). Essentially the students read through my solutions and give themselves feedback in purple.  I like this for two main reasons: It easily lets me see how much they completed (everything NOT in purple was before they looked at my key) and it lets the students see the level of work I expect to be shown.  Tomorrow during the first part of class, we will discuss the survey and which problems were easiest to solve graphically vs. kinematically.

General Physics:

I promised more images of The lens game, so here they are:

Given this set up, (Converging lens with a virtual image)

a student suggests: (the object, in orange, must be inside the focal length)

Here is the complete set.  The red arrows are real images, gold are virtual and there are even mirrors we will use later.

After this warm up, the natural transition was to quantify the relationship between where the object is in relation to the lens (object distance) and where the image is in relation to the lens (image distance)… in other words, we did the modeling the thin lens experiment.  With this group we stick only with real images and a converging lens.  That way we do not have to use a double lens system to locate the virtual images.   Tomorrow we whiteboard the results.

# Day 24: Graphical Solutions for cvpm Problems vs. Kinematic Solutions and The Lens Game

This was homecoming week, and homecoming week always culminates with a pep assembly.  This translates into only 42 minutes classes, except for those over our lunch mod — that mod stays the normal length.

So, what did I do in with only 42 minutes?  Well, I did not do much, but my students were pretty engaged.  I explained that they were going to work in small groups to solve 4 constant velocity problems.  Most of them I think thought “Four problems, 42 minutes, in a group , no problem”.  Most groups only finished 2 of them; not because they were super nasty, way too difficult, but because of two requirements I gave them.  Requirement number 1:  Every group member worked on every problem, so each student has his/her own solution, no divide and conquer.  If I am going to collect the assignment, it will be from one group member and I choose the one.  I use this technique often with group work because I think it fosters communication between the group members and it forces all the group members to be engaged. Requirement number 2:  The first two problems had to be solved using a graphical approach and a kinematic approach, and the last two can be solved either way, student choice. The graph could be a position graph or a velocity graph (though the position graph for these is much more appropriate) hopefully generated using Logger Pro.  After all, it is much more than just a graphing program.  This basic plan was developed with the help of my chemistry teaching colleague.

I really want my students to be able to solve problems both ways, it makes them better problem solvers.  Some of these kids (especially some in the more advanced math classes) tend to think they can just grab an equation and use it.  As you all know, some problems are MUCH EASIER to solve with a graph than they are with an equation or two.  The ultimate goal is to get the students to the point where they have a feeling of which approach to use.  Another added benefit is that if they get stuck going down one path, they can try solving it with the other method.  Likewise, if they are unsure of the answer, solve it the other way to verify it.

This idea of dual solutions is not really all that new.  The Minds on Physics-Motion book introduces it with constant velocity problems.  Others (Kelly O’Shea, Casey Rutherford and Mike Pustie) have given presentations and written about using a graphical approach to solve kinematic problems, usually constant acceleration.  I have also required it when we work those problems.  The new part for me was to use it with constant velocity problems. Now when we get to constant acceleration, maybe it will not be as much of an adjustment for the students.

General Physics:

The assigned individual practice last night was to finish the ray diagrams for the lenses.  To check the ray diagrams, we set up the optics benches. There are a number of reasons why we did checked it this way.

1.  Connecting the ‘pictorial representation’ to the physical set up and allowing the students to see the image.

2.  It reinforced the idea that we can see both real and virtual images without the use of a screen, just look back through the lens with your eye in the cone of light.

3. It introduced the optics bench and provided practice with it and locating images before we quantify it with the Thin lens experiment.

As we checked them, the students also completed a chart for their notes that summarized all the possible situations (6 total) for object locations and resulting images.  I see this as the linguistic representation.

In my long class, we finished the day off playing The Lens Game.  An awesome idea from a former colleague (Jeff Elmer) and member of our Share group.  I’ll snap some pictures on Monday when I warm the students up with it, but it is just some big (yet thin) wooden cut outs of lenses, images (real and virtual) and objects that have magnets on the back so they will attach to my whiteboard.  Then we play.  Given this object at this location,  with this lens, what image.  Or, given this object at this location and this image, what lens and where?

# Day 23: Wait, Which graph? and Ray Diagrams for Lenses

Today was a full-blown white boarding session.  A number of years ago, I slightly modified a Mind on Physics (MOP)Activity that provided a position graph with the motion of 5 objects depicted on it and a velocity graph with 5 different objects depicted.  Here is a picture of said graphs:

I like the set of questions that come along with it because most promote discussion amongst the students.  Here is my favorite:

Assuming object G starts at the zero position, at what time is it farthest from the origin, t3, t4, or t5?  It really makes the students think.  Today two different groups called drew the matching position graphs to help explain it.  It is the first glance at a changing velocity even though we do not mention the ‘a’ word.

Tomorrow we will throw some numbers at the constant velocity model.

General Physics:

Today was not the most exciting day, especially after seeing the aerial image yesterday.  We learned how to draw ray diagrams to explain (or predict) the images that form in lenses with the object at different object distances.  Pretty standard assignment I’m guessing.  To check the diagrams we ill use an optics bench to physically set-up the situation and allow the students to really see the image and discuss the characteristics.  There is more, but I’ll save it until tomorrow.