Category Archives: Modeling-workshop

Light Reflection, Refraction, & Diffraction

I hope it’s not getting old, but I would like to say that for those that have never attended a modeling workshop, although I hope this series of blogs peaks your interest, in no way should they be viewed as a replacement. Compared to the first set of workshop, I will not be posting links to the materials used as ASU and the AMTA have chosen to keep them under lock and key. If you are offended by that, I’m sorry, but they have chosen to restrict some materials, and I am in no place to challenge them. The basic thought process is that there research has shown that one best learns through doing, not through being told.  Therefore, the best way to learn how to implement modeling is to experience modeling for yourself, not to read about it.  In short, if you just read how to model, try to use it and find it ineffective, it could very well be you implemented it incorrectly.  Go to the workshop, you’ll thank me.  For those readers that have attended the Mechanics workshop, this may help you sort through the materials to which you have access, but again, you might want to try to get to one of these if you can.

2-D Wave Motion

We begin this portion of the unit by putting together ripple tanks.  Although these contraptions clearly show the behavior of water, I think there is a much cheaper solution.  By no means do I want to go full scale $2 whiteboard here (I mean no disrespect by that comment, I think that blogpost might be one of the greatest I’ve ever read), but I don’t see why you need that much complexity.  Why not just use a cafeteria tray, your finger and/or a ruler as a wave generator, and some simple objects (or even the edges of the tray) to show boundary interactions?  The time it takes to setup just doesn’t seem to justify the fact that you can see everything with a simple tray you’ve taken borrowed from your school cafeteria.

Anyway, mini-rant over, in using the ripple tank we are to look at how the waves react when a boundary is placed in the water.  Lo and behold, the waves reflect just like we saw with light!  We then try to look at refraction.  This part was very problematic.  The teacher notes say to put a thin piece of plastic in the water to change the water level.  For us, we did see a change in wavelength, however, we were never able to create a scenario in which the direction changed.  I think that side of things needs a little more refining (it has to be the procedure, it couldn’t be your humble blogger).

After a circle meeting to share the behaviors we saw, we worked on the second worksheet which brings in a new way of showing the behavior of waves: wave-front motion maps.  In some sense, just imagine a ray diagram with lines added to show the wave fronts (I’ll try to add pictures at some point, but the server here is way to slow).

One thing that we noticed in discussing this worksheet is that we needed to build the relationship between index of refraction “n,”  the speed of light in a vacuum, and the speed of light in a given medium.  I’m not sure if that jump to light is needed here, rather I think this might be better served taught separately as just mechanical waves (more on this to come in a later blogpost).

From there, we started a lab to show the diffraction of light.  The group set up a flashlight with a screen (open 3-ring binder) casting a shadow on a wall.  They then asked us, if a laser is placed under the flashlight and just touches the edge of the binder, where will the laser dot hit the wall in relation to the edge of the shadow?  We said that it should hit right at the edge of the shadow.  Once they set it up, we saw that the laser hit a few inches away, on the light side of the shadow’s edge.  I think care needs to be taken when setting this up to make sure that the students see that the laser is directly underneath the flashlight, and that the flashlight is small.  Several “students” (myself included) think that big of a shift seems unlikely to be due just to diffraction.

From there we move into a new behavior of light, that being diffraction.  By placing two boundaries in the water, we could look at what happens as the wave front passes between a small opening.  We were encouraged by the “teachers” to look at how the size of the opening effected the resulting diffraction pattern as well as how adjusting the frequency of the source on the diffraction pattern.

We finished up by working on worksheet 3: Diffraction to apply some of the concepts just developed.  One thing I really like was at the end of the worksheet.  The students are asked to predict what should happen if there are two openings.  After a good discussion, I left thinking, whatever the kids think here, as long as they can defend it using the wave model, is ok.  We now have a natural tie-in to a behavior of light not yet studied.  I’m ok leaving the question without a definite solution, as they’ll get it in the very next step of the unit, but that’s for the next blogpost.

{The Wave Model, brought to you by FIU, CHEPRO and the National Science Foundation}

1D Mechanical Waves

Once again, I would like to say that for those that have never attended a modeling workshop, although I hope this series of blogs peaks your interest, in no way should they be viewed as a replacement. Compared to the first set of workshop, I will not be posting links to the materials used as ASU and the AMTA have chosen to keep them under lock and key. If you are offended by that, I’m sorry, but they have chosen to restrict some materials, and I am in no place to challenge them. The basic thought process is that there research has shown that one best learns through doing, not through being told.  Therefore, the best way to learn how to implement modeling is to experience modeling for yourself, not to read about it.  In short, if you just read how to model, try to use it and find it ineffective, it could very well be you implemented it incorrectly.  Go to the workshop, you’ll thank me.  For those readers that have attended the Mechanics workshop, this may help you sort through the materials to which you have access, but again, you might want to try to get to one of these if you can.

As we ended the particle model unit, we had a good discussion about the overall storyline of this collection of units.  I think I’ll let this play out a little more, but I can’t help but think that this storyline is making it more difficult than it needs to be.  For those that don’t know, there is modeling material for just mechanical waves.  I haven’t had a chance to really go through them, but there are units for the oscillating particle model, 1D wave model, sound, and then 2D waves.  To me, that would flow very nicely out of the central force particle model.  With that in mind, I’m thinking you could just go straight into the wave model of light.

I’m fully aware that the current progression of starting with the particle model of light is true to an historical approach, but the major aspects of reflection and refraction could just as easily be first introduced with a wave model.  As you then move in to the photoelectric effect and the beginnings of the Bohr Model, you could then bring in the particle nature of light (yes, the photon model is distinct from a particle model, but what is the end product the kids NEED to know.)

Anyway, just some food for thought.  From what I can tell, this collection of three units is set up to be taught without teaching the mechanical wave units, thus this post will look at introducing mechanical waves to students.  To me, if you do include those units, you can skip this content and move directly into building the wave model for the behavior of light.

We begin this unit by asking the question, “What would you have to do to make a wave?”  You then provide some Super Slinkies or “Snakies” (or both) and have the kids go play.  By time they are done, your should be able to discuss the characteristic variable (frequency “f,” wavelength “\lambda ,” speed “v,” and amplitude “A“), what you need to do to increase the speed (effects of tension, amplitude, and distance on wave speed), and how non-conservative energy loss (due to friction) effects wave propagation.  If you want, you can pose these as questions to answer, or just let them play (and if need to send them back out to play a second time).  How much guidance would depend on the maturity of your kids.  One other important concept that you need to get across is that with waves, energy moves, even though there is not net displacement of the medium.

By connecting the Slinky to the Snaky, you can also begin to show that the speed of the wave depends on the medium.  If the students are really careful, by sending two quick pulses, they can feel that the frequency is independent of the medium.

The second major activity/lab is to get the students to see that the speed of the wave is a product of frequency and wavelength.  The modeling materials say to do this through the introduction of standing waves. To me, that might be too big of a jump.  Instead, I think you could have the kids measure the speed of a pulse  by measuring distance and time (possibly include various tensions) first, then bring in standing waves (and find frequency times wavelength) and compare to speeds found already.  Another option would be to video tape the waves and use video analysis to determine the speed of the wave.  In the end, with standing waves you need to build in the terms node and antinode.

During the standing wave lab, we had “major” issues trying to measure the variables using the snaky.  We later saw that the teacher notes say to use a slinky since it has less tension.  The other option is to use a wave generator which can better oscillate at a controlled rate.

At some point during this process you need to talk about reflection, interference, and superposition.  Student should see that both the wave model and the particle model can account for reflection (As you get to 2D waves, you can further show the behaviors of reflection discussed in the particle model).

From there, we completed worksheet #1 and later whiteboarded our results. If the kids didn’t notice it already, you can discuss transverse and longitudinal waves, along with terms such as crest, trough, compression, and rarefaction.

As mentioned at beginning, to me, this could just as easily be its own unit (most likely leading into another unit on 2D waves in water.

{The Wave Model, brought to you by FIU, CHEPRO and the National Science Foundation}

Particle Model of Light

First off, I would like to say that for those that have never attended a modeling workshop, although I hope this series of blogs peaks your interest, in no way should they be viewed as a replacement. Compared to the first set of workshop, I will not be posting links to the materials used as ASU and the AMTA have chosen to keep them under lock and key. If you are offended by that, I’m sorry, but they have chosen to restrict some materials, and I am in no place to challenge them. The basic thought process is that there research has shown that one best learns through doing, not through being told.  Therefore, the best way to learn how to implement modeling is to experience modeling for yourself, not to read about it.  In short, if you just read how to model, try to use it and find it ineffective, it could very well be you implemented it incorrectly.  Go to the workshop, you’ll thank me.  For those readers that have attended the Mechanics workshop, this may help you sort through the materials to which you have access, but again, you might want to try to get to one of these if you can.

Day 1:

We began today with a simple circle meeting (what one of our fearless workshop leaders calls the circle of truth) in which we introduced ourselves and shared a few war stories from the previous year.

After completion of the circle meeting, we returned to our lab tables (and told to begin “student mode”) and were asked to answer the question, “What is light” as a group.  Some of the groups were better than others at staying in “student mode” (yes my group was one of the bad groups), but overall we tried to reflect what we thought I students would give as an answer to that question.  We shared our whiteboards from our tables (there are only four tables, so we’re already in somewhat of a circle).  Overall, to me this question provides great insight into seeing what one’s students already know about the topic.  You can see how much they remember from physical science and/or chemistry (depending obviously on the sequence of classes at a given school).  One of the interesting side questions that came up was where would these three models fit into the overall flow for the year.  We agreed we’d come back to that later, but the question stands, should you teach mechanics -> light -> E&M, or mechanics -> E&M -> light.  One other side note that I thought was interesting was that LCD Projectors project Red and Blue light in one polarization and Green in the other.  If you’re able, you can show kids this by using a big enough filter for your class setup (assuming you have an LCD projector).

After the discussion came to a close, the next major task (and I do mean major) was the Light Concept Inventory.  By no means do I mean to disparage whoever put in the time to make this, but it was much more of a bear to get through than the FCI.  They gave us an hour and forty-five minutes and most of us weren’t done.  I’m not sure if that is more about us, or just how long that test is.

From there we got started with some of the introductory hands-on stuff.  Each table was given a bucket of equipment with a small maglite flashlight, a candle, a torch lighter, laser pointer, and a small fluorescent light bulb (vertical coil not spiral).  We were each given a small whiteboard (about the size of a piece of computer paper, and asked to individually write down what we noticed about light as we messed around with the contents.

On a side note, our fearless leaders told us that having individual WB’s allows each student to have their say, then let the group talk, and either combine onto one big whiteboard, or just share individually with the class.  Since my school is going 1:1 with iPads, I’m thinking I’ll just find a whiteboard like app and do this step using the iPads.

During the subsequent discussion, we developed some working vocabulary (through socratically led discussion) such as luminous objects vs illuminous objects, a working definition of a shadow (which in the next activity we modified to distinguish between umbra and penumbra shadows), and transparent, translucent, and opaque objects.  We also agreed on how to best represent how light travels -> rays. One of the cool highlights was spraying something (I didn’t catch what it was, but it looked like a can of hairspray or spray deodorant) in the air along the length of the path of a laser pointer, think classic laser scenes from Mission Impossible type movies.  As I said, not sure what it was, but the particles stayed in the air for a very long time, making a very cool effect.

After that, we were told to take the “shroud” off of the maglite and use it as a base, in so doing creating a nifty point source of light.  Again, we were told to play around and on our miniWB’s write down what we observe with respect to the formation of shadows.  After some playing in the dark, they brought a few extra maglites to each group so you could make shadows with more than one source of light.  We again shared our findings, during which we made the distinction between umbra and penumbra.  Some highlights of the discussion were me making the classic barking dog shadow puppet on the wall and using a string to reinforce the linear path of light by stretching it from the light source to the shadow on the wall, just touching the “students” finger along the way.

From there, we made two different types of pinhole viewers.  The first was just a Styrofoam cup, a piece of wax paper, and a rubber band to attach the two.  Pull the wax paper across the opening and secure with the rubber band.  Then punch a small hole with a push pin.  The second was a little more advanced, using two pieces of PVC pipe (one of which just slides inside the other).  Put aluminum foil across one end of the larger PVC pipe and wax paper around the narrower one.  Attach both with a rubber band, and poke a hole in the middle of the aluminum foil.  Point either at a light bulb with an arrow (cut I’m guessing with a Xacto Knife out of electrical tape) and the leftover piece of tape with the missing arrow) and observe.  (I have a picture, but I’m having trouble getting it to upload, so I’ll try to add it soon)  Again, we wrote down our individual thoughts on the miniWB, shared with our lab group, and later with the entire class.

At the end of the first day, we recapped what we know as to the basic characteristics of light and discussed what might be able to explain them (a stream of particles).  They let us know that starting tomorrow, two of us would be the “teacher” for each of the coming sections in the rest of the unit.  They posted on the board what part (whiteboarding worksheets or leading the next couple of investigations) we were to get ready to lead.

Day 2

Our homework for the previous night was to complete the first three worksheets of the particle model packet.  As we began setting up our whiteboards, we had a little bit of a side discussion.  So of the big things that came up were: 1) To keep in mind that although modeling is unstructured, it is still very goal oriented.  Make sure, as the teacher, you have looked over the materials and especially the teacher notes, to make sure that, no matter the path your class takes, you get them to the correct destination. 2) Make sure you are continually helping the students to extract the core concepts, one way to do that is to make sure that you take a few minutes at the start of one day/beginning of the next to summarize the concepts or important observations that are needs moving forward within a given unit.  One other thing I offered was Kelly O’Shea’s awesome Mistake Game as a way to improve discourse during whiteboarding. (Kelly just made an updated post to this awesome way of whiteboarding.  Definitely check it out.

A couple of the key ideas that are beginning to emerge for this unit are after our introductory interactions with light and having completed the first three worksheets:

  1. Light can be modeled as particles that move very fast in straight lines.
  2. Light can bounce off of objects and take a new straight path.
  3. There are too many rays emitted by a source to count.
  4. Some objects create light, some block/reflect light, some allow “all light” through, some only partially allow light through.

The readings talk about a pinhole making a reproduction, not an image, but so far, we’ve been just using the term image.  I think the official packet is trying to make the distinction since at “all locations” the reproduction is visible, while an image is only visible at fixed locations due to the geometry of the optical system.

After whiteboarding the first three workshops, we than began the paradigm lab for this first unit on light.  From here, 2 of the participants are now running the show, so we get to lead the group of misfit “students.”  (By the way, in my opinion, the worst students are teachers.  We tend to break down and do all the things we find so annoying too quickly.)  The leaders for this section began walking through the tradition modeling introduction to a lab: what do you notice/see, what can you measure/change/control, what can you manipulate.  If you’ve read this far, I’m sure you’re somewhat familiar with the process, so I’ll keep this somewhat simple.

The equipment we were eventually given was a meter stick with support stands, a square piece of paper and clips to attach it to the meter stick, a square piece of paper with about 400 dots punched out (approximately have the diameter of traditional 3 hole punch dots), a mini maglite, and a ring stand w/clamp to hold the maglite.  At some point, I’ll get a few pictures imported, but so far I’m having trouble doing so with the FIU guest server.

The overall goal was to compare the number of dots seen within the square screen as a function of distance to the source (as a group, we didn’t make a good distinction about distance to source or to the dot paper, the groups that measured to the source had much more reliable results).  Just about all the groups acquired data that looked like an inverse relationship.  As we then linearized the data, some groups had data that then looked parabolic and linearized again.  During the whiteboard discussion we compared results and procedures to try to flush out the correct relationship.

The original plan was to then whiteboard the fourth worksheet and have a board meeting, however the workshop was interupted due to the, at the time, upcoming announcement of the Higgs Particle.  FIU has a few particle physicists involved in the CMS project at CERN, so one of them gave a pretty substantial preview of the coming announcement from the other side of the pond.  As a side note, we are planning to have a Higgs party with them at 3 am!

Update: Higgs party was awesome in a nerdly way.  Hearing room full of scientists gasp at a little bump on a curve and then give a huge applause at the mention of 5\sigma, was quite an interesting moment.

Day 3

We began our third day with whiteboarding worksheet 3 from the modeling packet.  Some interesting thoughts that came out of the discussions were as follows.  One misconception that were battling over was whether or not students would think that decrease in light intensity as you move away from the source would be due to light spreading out or due to absorption (meaning a loss of energy do to some non-conserved force).  One of the problems led to a great discussion of the concept of a point source of light emitting light in all directions vs light in a beam (laser light).    One of the groups did an awesome job of explaining the solution to one of the problems by explicity starting from the relationship derived in the first lab (I\propto \frac{1}{d^2}).  During that discussion, we also developed the idea that intensity of light is like a “density” of light rays.  A few other tricks of the trade that were discussed was the idea that we need to reinforce the idea of mathematical symbols.  Although we got a little side track nitpicking on the math teachers, the important aspect of the discussion was that we can cause confusion by calling any independent variable x part of  the time and using x for only position other times.  We need to try to stress that the axis are horizontal and vertical, not x and y.  We also need to continually push our students to try to figure out the meaning of the slope of a graph, not just jot the numerical value down.

One absolutely fabulous moment, in this bloggers humble opinion, was during a discussion of how long it takes light to get from distant stars to Earth, and is the light from “now” or from the past.  To keep it short, just watch this:

From there, we moved into another lab, images from a flat mirror.  The group began by showing a flat mirror and a laser, and asked what we noticed the laser was moved around while being pointed at the mirror.  Keeping the story short, we set out to find the relationship between the incoming angle and the outgoing angle for reflection in a flat mirror.  To help show how awesomely powerful Kelly’s Mistake game (I don’t know if she invented it or not, but she introduced me to it and has given numerous blog posts explaining it).

I convinced my group to measure all of the angles relative to the mirror on the side of the mirror on the incident beam (Meaning a beam coming in very close to the mirror is 10^o and the reflected beam is 170^o)  At first the other groups were baffled that we didn’t measure from the angles from the normal line, but it lead to a great discussion of how to justify the convention to measure from the normal line.  Two great lines of questions that eventually emerged, do the ingoing (we haven’t really stressed the term incident so far) rays and out going rays look symmetrical?  If you agree that they do, should we have convention of describing the angles that stress how symmetrical they look?  Which was then followed up by “Where do you normally stand when you look at yourself in the mirror?”  If that’s where you normally stand, isn’t that a great reference point?  To me, that might be the best reasoning to call it the “Normal Line” I’ve ever heard.

After that, we started building towards developing a way to find the image in a mirror.  They gave us this awesome red pseudo-mirror (You can get one here).  Draw something, most of us just drew a letter, then trace over what you see on the paper/mini-whiteboard behind the red pseudo-mirror.

Another great discussion then ensued in which we tried to figure out how to convince students that the image was behind the mirror, not inside/on the surface of the mirror.  One great point was to get something with small writing.  Stand at a distance where it can be read, and where twice the distance it becomes illegible.  Then stand at that distance from the mirror, and try to read the writing.  One other suggestion was to get a manual focus SLR type of camera (most likely using the manual setting, since all of them come with auto functions).  Measure how far away you are from the mirror, then look at the distance the camera is reporting when your image is in focus.

A second part of this side discussion was over trying to explain why images “flip” or “reverse” side to side but not up and down (seen with the pinhole viewers).  What Jeff suggested was to write on wax paper.  Hold it so that it looks normal and then look at what you see in the mirror.  (If you really need to convince someone, then turn and face them and ask them what they see {text is backwards for them but normal for you [yes I love parenthetical comments]}).

Once we got back on track, we began whiteboarding the next worksheet (#4A).   In the process, we summarized two laws for reflection:

  1. The normal line, incident ray, and reflected ray are always in the same plane.
  2. The incident angle is equal to the reflected angle.

We also began to distinguish between reflection off a “rough” surface (which we eventually defined as diffuse reflection) versus reflection off a “smooth” surface (specular reflection).  We also began to wrestle with the terms real versus virtual images.

One great quick take home assignment/quiz that one of the workshop leaders uses is to have the kids go home and look at their face while being very close to a mirror. Using either a whiteboard marker (lipstick, tape, or any other removable form of making a couple of marks), make a line at the top and bottom of where your head appears to be on the mirror.  Then answer the question, what happens to the position of the top and bottom of your head relative to those marks as you walk backwards (away from the mirror).

The last part of today’s class was spent with a paradigm lab to study curved mirrors.  This was the part in which a fellow workshoper and I became the “teachers” for the class.  We began by setting up a candle in front of three mirrors at slight angles to each other (similar to what you see in dressing rooms).  We then directed a laser, from in front of the set up, at one of the mirrors such that it hit the back of the candle, and asked the class how does the laser get to the back of the mirror.  We then asked is there another place we can direct the laser such that it will still hit the back of the candle two times (answer move to the side so the laser hits a different mirror).

The reason being that we wanted the class to recognize that, since the mirrors were not coplanar, they had normal lines that were not parallel to each other.  Thus, the “same” path of light behaved differently due to different normal lines.

We then asked, how would the setup change if we had several smaller mirrors, each at different angles around the candle (set up in an arch).  To which, they said, there would be more ways of directing the light to the back of the mirror.  From there we showed them a concave mirror, and ask, “So then what should this mirror do?”

We then showed them how, but putting a triangular piece of tape on the “window” of a flashlight we could project an image of that triangle onto a surface.  We then did a quick hand waving of the modeling series of pre-lab questions to get the class to go about looking for a relationship between the distance from a candle to the mirror and the distance from the image to the mirror.

As the groups were working, many quickly saw that the data had an “inverse” look.  However, when they tried to linearize by taking the inverse of one data set, many were unsatisfied with their result.  As per the notes in the teacher info, we help most of the groups  by telling them to take the inverse of both data sets.

At this point we had two cool ways of finding the focal point.  The first was that we went outside with a big classroom demo size concave mirror and a piece of balsa wood.  (I’ll eventually upload a video, but I’m not going to bother trying to do so with the limited bandwidth I have here at FIU).

We also used an overhead projector to create a beam of light.  After turning off all the lights in the room, we put one of the small concave mirrors used in lab in the path of the beam.  We then got the special effect spray again and could very clearly see the cones on each side of the focal point.  After quickly measuring that point, we were ready for the whiteboard/circle meeting.  Most of the groups seemed ok with having the graph of \frac{1}{d_{image}} vs. \frac{1}{d_{object}}  Just about all the groups agreed that the slope was “-1.”  We then asked what do you notice about the vertical or horizontal intercepts.  Most quickly noticed that they had the same value.  After a little prodding, they realized that they were each the inverse of the focal distance we just measured.  So I then showed them (if we had more time I would have had them do it) the equation of a standard line: y = mx + b and then began replacing the variables with what we had in our graphs.  Since the vertical axis is “\frac{1}{d_i},” that replaces the “y.”  Since the slope is “-1,” that replaces “m.”  The “x” gets replaced with “\frac{1}{d_o},” and the “b” with “\frac{1}{f}.” Thus:

\frac{1}{d_i}=-\frac{1}{d_o}+\frac{1}{f}

Which if you so choose can be rearranged into the more standard form:

\frac{1}{f}=\frac{1}{d_o}+\frac{1}{d_i}

Day 4

We began the day whiteboarding worksheet #4 from the particle model packet.  Some important concepts from the were the 3 principle rays (which we said would need to be discussed before the students would be able to do any of the worksheet.  One significant flaw we saw was that the first two drawings on the worksheet did not have the mirrors drawn accurately (all others looked fine).  Due to this, if students tried to complete the worksheet using protractors and the (2nd) law of reflection, they would get a completely different result than if they used the principle rays.  Thus, if and when I use this worksheet, I’ll probably cut and paste from somewhere else in this worksheet.

We also said that somewhere during the curved mirror lab, you need to relate the focal length to the center of curvature (a few of the problems provide one but not the other, which might cause problems).

One great rattle mantra that one of the workshop leaders says he uses is, “If it diverges on one side (of the mirror/lens), it converges on the other.”  I thought that was such a great statement, since I always have to battle the confusion that  “diverging rays don’t intersect.”  Meaning, I often have students that get diverging rays and say that they don’t intersect.  If you can get that saying into their head, they will know to look on the other side of the mirror/lens.

From there, we were given two paper cups with magnets glued to the bottom (doesn’t have to be magnets, just something somewhat flat).  We were told to set them up so we could just see the magnet’s edge, then begin to fill one with water, and write down our observations (on a mini-whiteboard).  We were also given two clear cups with a pencil in each.  Again, we were told to fill one with water, and write down our observations.  After which, we discussed what we noticed, in so doing, beginning the notion of refraction (I’m not sure if this is necessary if you’ve used the mechanical wave set of units as they also build the idea of refraction).  One cool demo they did was using a laser pointer, a thin rectangular water tank (with a few drops of coffee creamer in the water), and the laser fog spray mentioned earlier.  They eventually had the tank hang over the edge of the table so you could get the laser entering the top and leaving the bottom of the tank.

From there, we were shown a small semicircular water dish, and led to the objective of the lab: to determine the graphical and mathematical relationship that exists between the ingoing and exiting angles from the water dish.

One significant flaw flaw with this step is the fact that, with this equipment, every group got data that would be likely misinterpreted as linear.  The teacher notes say to lead the students (either before or during the lab) to compare the semi-chords (relative to the normal line) to lead them to the sine relationship.  We all agreed this seemed very forced.  A few other things we discussed was that if you use error bars in the graph, you very easily see that the line doesn’t fit the data.  However, most of us do not use error bars, believing it’s a level of sophistication above (and not necessary for) the introductory physics students.  In the end, we all thought that if we sufficiently laid the groundwork earlier in the year, most of our students would see that the data can’t be linearized with any of the tools they’ve developed so far (squaring or inverting one set of data).

To me, this is a similar stumbling block as what occurred during the curved mirror lab (having the students invert both sets of data).  What pops in my head for each of these cases is the religious “poem” “Footprints in the Sand.” I think this is where the idea of physics coach/guide (not teacher) becomes a powerful image.  I think this is an example of knowing where a major stumbling block will prevent the success of our students.  In such cases in which they can’t get there easily on their own, that’s where we step in and offer direct instruction.  As a side note, I would not cite the poem explicitly in class as I don’t want my students to infer that I’m a physics god.

One other great idea that came out of this discussion, that I had never heard before, was the “least time” argument. It was described/introduced using the following “smart dog” problem:

Imagine you are playing fetch with your dog at the beach.  Your dog is a certain distance down the shore.  You throw a ball directly into the water in front of you.  Since the dog can run faster on the sand than swim in the water, what is the fastest path for the dog to take to reach the ball.

Here’s a link to a similar problem.

From there, we began working on worksheet 5A, which we will finish discussing tomorrow.

We began Monday’s class by finishing worksheet 5A on refraction using the particle model.  One interesting idea that came out of it was a way to show the connection between concave mirrors and converging mirrors.  What someone mentioned was to take the piece of paper  with a mirror and fold it along the middle of the lens.  What you can see through the paper is a concave mirror (similar result for diverging lens to convex mirror).

One other discussion we had was over the idea that refraction creates an “image.” If you draw two rays reaching two different “observers,” you can extend those refracted rays to one intersecting point.  We tried to figure out if that refracted “image” location.  After a brief attempt to be like one of the physics blogosphere’s super physicists (Rhett Allain), I made an excel spreadsheet that calculated the apparent position based on geometry, and was able to show that the location is not consistent.  If I ever get the chance, I’ll try to make a post about that. If Rhett beats me to it, I’ll post a link.

One of the important concepts that comes out of this worksheet, is the critical angle.  I’m not sure if I would need to give a little bit of a heads up on this, or just let it play out during the whiteboard meeting.

From there, we moved to worksheet 5b, which now builds on the particle model through quantitative questions.  One of the interesting discussion came up when asked about the difference between an image and a reproduction (pinhole camera/viewer).  Since as a group we used them interchangeably earlier in the unit, there was some major confusion here.  I would definitely recommend stressing reproduction earlier (and why it’s not an image then).  You’ll probably still have some misconception at this point, but probably less.

We also had a great discussion about how to lead students to understand that, when placing a screen over have of a lens, you can still see the entire image.  One slight flaw with the worksheet is that it uses a “large” lightbulb, so a ray of light cannot directly reach the bottom of the lens (without passing through the lens itself).  I might alter this problem to show a candle instead to avoid the confusion.

We finished the unit by discussing the “Essay Questions.”  For this unit (and the other light units) students do not write lab reports, but rather write reflective essays.  These essays ask the students to reflect on what the particle model is, and what evidence he or she has for each aspect of the particle model.  The second essay asks the student what aspects of the particle model seem incomplete or unable to fully explain behaviors of light.  The student is then asked to propose a modification to the model or a chance to begin to create a new model all together.

One great demonstration that came out of the talk was to take a squirter bottle.  You can create drops of water to show a “particle” and slowly squeeze until the drops become a stream.  It is very simple, but yet, a great visual to show one particle becoming many.  One other great visual was to use a can of spray paint or hair spray to show particles leaving a flashlight.  This could also be used to model the intensity of light.  You wouldn’t spray paint on a wall from several meters away, no paint would reach the wall.

Well, that’s it, that’s the particle model.  I’m guessing this might not give an absolutely clear picture of the model, but I don’t want it to.  I want this to be enough to help those that have attended to remember

{The Particle Model, brought to you by FIU, CHEPRO and the National Science Foundation}

Modeling Unit 8

Unit VIII: Central Force Particle Model

Jon started today by giving a brief demo.  He had a rubber stopper tied with a string attached to a hanging mass.  In between the two was a plastic tube (think very sturdy straw), which he held in his hand.  He asked us where he would need to release the ball in order to hit a certain object.  He then asked where he would need to release it to hit a different object, in a different part of the room.  He then socratically questioned us to say that the speed of the rotating stopper was constant, but the velocity was continuously changing.
He then asked us, what causes a change in velocity? (answer: unbalanced force)
What direction must the force be? (answer: towards the center*)
What direction is the acceleration? (answer: towards the center)
*Chris showed us a demo we can do if the class doesn’t agree that the force/acceleration is towards the center.  He grabbed the bowling ball (yes, the bowling ball, again) and a broom, and asked one of the students to make the ball move in a circle.  She first started out inside the circle of the ball and was constantly pulling the ball towards herself.  Chris then had her stand outside the circle and put a cup as a reference point for the center of the circle.  She again had to constantly push the ball towards the cup.
From there, Jon led us to derive an equation for the average speed of an object in circular motion:
\large \overline {v} = \frac {\Delta x}{\Delta t} = \frac {2 \pi r}{\Delta t}
From there, we walked through the tradition questions for paradigm labs:
What do you notice?
What can you measure?
What can you manipulate?
Then Jon helped us to create the purpose:
To determine the graphical and mathematical relationships that exist between the speed of the stopper and the amount of mass hanging on the string.
{We did not study the mass rotating, however you could have part of the class investigating this, and the radius if you want to “kick it up a notch” – Bam!}
We found that this lab was very tricky and had lots of error.  A couple points to minimize the error:
  • Have the lab members keep one job: timer, recorder, twirler
  • Make marks on the string to help see where it needs to be to keep a constant radius
    • This is huge!
  • Possibly use a force sensor held against the table.
  • Possibly use video analysis to determine the actual radius
    • as the ball drops, the length of the string is no longer the true radius
    • cut a slit in a tennis ball and squeeze over stopper to make a more visible point.
We also discussed what to do if a group has “bad” results.  We agreed that early in the year, make sure you are doing a thorough job of checking the groups while they are experimenting to avoid this.  However, as the groups get comfortable with the whiteboarding process, letting mistakes slide into the meeting can make it more interesting.  Think through when you want to call on those groups.  We also agreed that we need to remind students that the data measured isn’t wrong, the procedure to keep multiple variables may have been insufficient, but the data is the data.  Encourage the students to discuss the subtleties of their procedures to determine where groups differed.  If the class is getting bogged down, don’t be afraid to say, “Let’s come back to this after all the groups have presented.”
If the students didn’t already, have them create graphs of F_{hanger} vs $v^2$ instead of m_{hanger} vs v^2.  When they do so, ask what the slope represents.  If they aren’t sure, ask what the units of the slope are (kg/m).  Since the slope is constant, what mass and distance are staying constant?  To which, they should reply the mass of the stopper and the radius of the circle.  From there you should be able to derive the centripetal force equation:
\large F_c = \frac {m v^2}{r}
When we came back from lunch, Jon again attempted to shoot his ping pong launcher.  See the results in this blog post.
After that, we began work on Unit VIII worksheet 1 & worksheet 2
A couple great ideas from one of our cohort to help students “see” circular motion:
  • Cut a wedge out of a disposable pie pan, then roll the ball roulette style
    • ball will come out in a straight line
  • Have student’s run down multiple flights of stairs as fast as they can
    • may need to make this a “mental” experiment not an actual one.
    • ask students what they must do to turn from one flight to the next while at a landing
Before the workshop started today, I saw a very cool video on youtube that I’ll show, just because I thought it needs to be seen:

We started to day whiteboarding our summaries of Arons’ Chapter 5.  For those that haven’t read it, it’s a fantastic book with sharp insight into the shortcomings of teaching physics.  It’s written at a very high level, but once you get used to it, it has a lot to tell you about how you should be teaching physics.

From there we finished up Unit VIII
What worked?

  • We liked the demo with making the bowling ball move in a circle
    • Especially the person outside the circle
  • Getting insight into what to do (and what not to do) during a lab
    • once members determine the job they can do, stick with it
    • POGIL
  • Student discussions help them get understanding as to what lab was showing
  • The idea that data isn’t wrong, the method of isolating variables may not be sufficient
  • The fact that we (the students) are always finding the graphical and mathematical relationships
    • once you get the hang of it, you know what to do when the models get more difficult
    • new lab, same analysis

What didn’t work?

  • Teacher notes require editing/more detail on graphs
    • Centripetal force lab

Notes:

  • Even though we knew what the outcome should be, struggling through labs is very helpful
  • For labs that fail (class completely lost), come back as a teach demo and explain how you are doing the experiment differently
    • demo vs lab less time if you don’t have it (due to lost period of failed lab)
  • If you have problem students or limited supplies, split the class and have half do the lab and the other half work on problems & switch part way through.
  • Use record player and put a thin piece of wood (less than 1×4) across the deck, have students measure coefficient of sliding friction \mu_{k}, and predict what is the greatest radius to place the penny such that it won’t slip. (Find \mu_{k} from maximum angle with no slip).
    • vernier has a lab for accelerometer and turntable
      • difficult to due with calculators, not too bad with computers
  • Would be nice to see a paradigm lab for universal gravity
  • One member mentioned that this graphical analysis is very important as the next generation standards will implement a lot more graphical analysis.

Modeling Unit 7

Unit VII: Energy

Jon and Chris then started the paradigm lab by asking for 3 volunteers

  1. Held a bowling ball and walked at constant speed
  2. Pushed against a wall
  3. Lift a small mass

Group was then asked, “Who is doing the most work?”

Physics defines work in a more specific way

A change in position due to a force that is applied in the direction of the change in position

-Establish direction early on and physics specific definition of work

-Student “1” does no work on the bowling ball

-Pushing a stuck car – you should push parallel to maximize work
-Teach students the concepts before introducing the math

Jon dropped a bowling ball – had cohort brainstorm different types of energy.
Discussed the energy transfer mechanism -> work

“We” then began Unit VII worksheet 1 before doing any labs.  Worked on the assignment individually and then presented a problem on whiteboards.

#3) still has velocity at the top
Discussion of energy as a scalar

Be careful to use “transfer” instead of “lost” when referring to energy

Jon and Chris then used a Piece of equipment with four wooden track, each with a different shape and unique color  (the only similar product I’m finding on the internet is this).

The students are then asked,

  1. “Which ball will reach the end of the track first?”
  2. “Which will hit the ground the farthest from the table?”

Answer to #1 – ball on the “blue” track  & #2 – all are the same except the “yellow” track

Then moved on to “Spring Lab”
Mass hanging on a spring, which is hanging from the Force sensor
Purpose: To determine the mathematical and graphical relationships that exists between force and displacement of a spring

Each group was given 2 different spring (1 short & 1 long)
{Overall there were 2 different lengths and 2 different spring constants for this lab.} 
{Some groups randomly selected 2 lengths with same k, others had 1 of each k}

Group plotted F vs x  results on whiteboard

Chris took us on a tour of the ASU Modeling website.  Most of the important stuff he showed us is password protected.  For those that are reading this that have not attended a workshop, sorry, I can’t help you.  Chris showed us some of the math resources he uses to help students with trig/vectors.  Since there are several teachers present that also teach chemistry, Chris showed us some of the chem resources as well.  One thing we discussed was using flame tests or emission tubes to show the quantized model of the atom.  Someone asked about diffraction glasses, so if that person is reading this, go here. Chris also showed us two important inventory tests that we can use as pre- and post-tests to assess our students understanding.  One was the Force Concept Inventory (FCI) (Mechanics) and the other was the TUGK2 test (graphing).

After the tour, Chris also mention a book to us that he has stumbled on due to modeling that he has found to be very informative: Preconceptions in Mechanics.

Jon and Chris also mentioned joining the Modeling Association and the American Association of Physics Teachers, as they both have a tremendous amount of materials for physics teachers.

Before we got into the heavy stuff again, Jon also showed us a great website with lots of demos: U of Minn Demos.

From there we began to discuss the lab from the previous day (Hooke’s Law Lab).  A few of the key points that came up we that we felt that this was great opportunity to discuss the limitations of a model, namely the fact that the spring will not always be a linear relationship.  Most groups, due to the strength of the spring also found that the beginning of the plot (near the origin) was also a non-linear relationship.  Other important questions the were raised, such as, “Did the length of the spring effect the spring constant?”

If the groups followed traditional graphing protocol, they would have plotted \Delta x vs F, which leads to a great series of questions.  What does the slope of the graph represent? What does it mean to have a bigger slope on the graph?  How can we manipulate the graph such that an increase in slope means a stronger spring?

You can also possibly delve into significant digits.  What is the variation/uncertainty in the applied Force?  What would that do to you calculation?

Jon also mentioned, that if you have the resources/equipment, set up the experiment with both the force probe and the motion detector, so even if the spring is bouncing, you can get F vs \Delta x data.

From there, we began working on Unit VII worksheet 2.
A few things to note:
#5 This problem is a great reminder of the graphical derivations from kinematics.
Specifically the derivation of the area when you know the slope of the line
See derivation of \large \Delta x = v_o t + \frac{1}{2} a \left(\Delta t \right) here

From there Jon tried to create a demonstration, however he was missing some necessary materials.  Here’s a list of what you need (not what he had):

  • 1.5″ PVC pipe (Jon uses an 8 ft pipe, but shorter is ok) (clear tube if you can afford it)
  • 1/2″ drill bit (to make a hole drilled about 2″ from one end of the PVC pipe)
  • 3/8″ hose barb (something like this, may need different size depending on vacuum tubing)
  • Teflon tape (wrapped around barb before it is screwed into 1/2″ opening in pipe)
  • 40 mm Competition Ping Pong Ball (as we saw, the basic/cheap ones won’t work)
  • 3″ packing tape
  • Jon also mentioned you may need a coupler on each end for added surface area
  • Soda can (with a book on top for added inertia)

So far Jon hasn’t gotten the demo to work, once he does, I’ll post pictures/videos.
{Update 7/13: Here’s some pictures and videos taken during today’s successful launches)

While he was tinkering to get that to work, one of the cohort near me was talking about a cool demo she does with her class.  She gives the kids garbage bags (unused) and asks who can inflate them with the fewest number of breaths.  Once the kids are about ready to pass out, she shows them how you can do it with one breath (Here’s a great set of resources, if you scroll down until you see pg 13 in bottom right corner, you’ll see the explanation.)

Once Jon conceded that he wasn’t going to get his demo to work today, we moved on to another lab.  The set up was a modified version of Option 1 of the Energy Transfer Lab in the Teacher Notes (see bottom of page 8 of the notes) in which the track was on an incline.  By adding this twist, you can show the transfer of energy from elastic to kinetic to gravitational energy.

We again worked through, What do you notice? What can you measure?
Chris then briefly showed us this:

Before continuing with then circling/striking out what we can/cannot manipulate.
From there, we stated the purpose:
To determine the graphical and mathematical relationships that exist between the initial starting position, the launch speed, and the maximum height.

We ended the day experimentally determining the spring constant for the metal loop.

We started today by finishing the modified lab.  After finishing, we all made whiteboards of our results and presented them.

During the presentation, a few ideas came up.  One, the groups that used the motion detector had much better results than those that measured the compression with their eyes.  Two, instead of measuring the spring constant with hanging weights separately, we could attach force sensors to the top of the car and measure the force directly during the launch.  Third, we could use a level app from smart phones to measure the angle of inclination of the track.  Four, we could use video analysis to measure the change in height (although I’m not sure if this would be as accurate as the motion sensor).

In the end, adding an inclined ramp to this lab, definitely increase the level of difficulty.  I think this would be good for a second year class, or possibly AP.  However, I think adding studying 3 forms/modes of energy in one experiment is a bit too much for first year students (especially standard level).

One of the groups placed their energy pie charts on a sketch of their velocity vs time graph, which proved to be a great way of showing the energy relationship (most of us just made a pseudo-motion map with a sketch of the track).

One other piece of advice from Jon was to make sure that you stress energy “transfer” not energy “loss” when discussing friction or other losses of energy due to non-conserved forces.

Jon also mentioned that, surprise-surprise, he had a homemade launcher instead of buying the circular metal spring.  He took a piece of 2×4 and attach two 16 penny nails (far enough apart to rest the track in between the nail).  Once the track is place perpendicular to the wood, in between the nails, he stretches a rubber band (new each lab) between the nails (over the track).  Here’s a rough sketch of a top view:


Where the yellow oval represents the rubber band, the blue circles are the nails, the grey rectangle is the track and the brown rectangle is the 2×4.  If you need to keep it level, just add a 2×4 to the other end of the track.

From there we moved to a paradigm demonstration for “potential” energy.  (I have it in quotes as we were told this name can carry with it bad misunderstandings, instead you should just call it gravitational energy or elastic energy, etc.)

Jon said that “energy” can cause pain.  So he had Chris come to the middle of the room (simulating a student from the class).  He told Chris to stick one foot out in front of him, and then asked, “Would you rather me drop this bowling ball (from waist high) or this tennis ball (also from waist high)?”  Obviously we were all cheering for the bowling ball.  Chris then asked, “Would you rather me drop the bowling ball from here (waist high) or from here (just above his shoe)?”  Chris then asked us, do you really need to do anything else to teach $\Delta {E_g} = mg\Delta h$?  Then (just to remind us of the spring equation), he suggested having 2 rubber bands, and basically run through the same thing, which rubber band would you like to have snapped on your arm, and from what distance?

After that we started working on Unit VII worksheet 2b.  Like most of these, we worked individually and then each group was assigned one problem to whiteboard.

During the board meeting we had a great discussion as to exactly how energy flow diagrams and energy bar graphs should depict the drawing.  One part of the group felt that if the type of energy is known, it should be identified (even if the interaction is outside the system); others felt that if it wasn’t part of the system, it should not be named.  I’m not sure who “won” the debate, and we basically left it up to each person to use as he/she sees fit in their class.  During the discussing, it was pretty obvious that even the experienced teachers in the room had some misconceptions about energy and what it really means to define a system.  We agreed that this is a tricky concept, and talked about to what level of understanding we should try to get our students.  Is it enough for them to merely identify the types of forces present and just that energy is entering/leaving the system, or do they need to describe the the exact means by which the energy is leaving (form of heat or work).  {My guess is that in the end it depends on your students and the standards/goals for your class}

One thing that came to mind for me was my Thermo I&II teacher who stressed that if it’s not important enough to be identified as part of your system, the interaction doesn’t deserve a name.  I’m also well aware that my students are not sophomore engineers in a Thermo class but 1st or 2nd year high school students.

We then went on to discuss our reading from last night, Making Work Work.  We did a different style of discussion in which each group wrote down 3 things they felt important within the article and then we shared our thoughts.

We finished the day by wrapping up Unit VII
What worked:
After we go the hang of them, we liked the energy bar graphs and flow diagrams
We liked the lively discussion over worksheet 3b
We liked the chaos/challenges of the last lab (cart on the incline w/spring launch)*
We felt that when Chris showed the graph he expected, we better understood what to do**
We liked struggling through the lab, it gives us a better appreciation for what I students will experience

What didn’t work:
We realize that we need to be reading the “readings” provided to the students, so we know what “they know” for each lab.
We felt that the prior knowledge requirements/level was too high for the last lab*
We felt that same lab did not have clearly defined objectives**

* and ** comments show just how split we were for the lab

Jon, Chris, and David Jones (the FIU instructor who helps facilitate this workshop) talked a little bit about the fact that the binder and online resources are not a script we have to follow, rather the tools that have emerged from numerous teachers struggling with this style of teaching.  They encouraged us to use what we liked, and modify or omit what we didn’t.  In essence they reminded us that we are professional teachers who know our students and school culture.  One of the great characteristics of the modeling method is how easy it is to adapt things to suit a given school.  As we grow in using some or all of this material, we were encouraged to share our take on it with others, so the material continues to evolve.  Their biggest hope was that we didn’t just copy the binder as is and pass it out to our students.  I think the biggest advantage to coming to this workshop is beginning to find how I might use all this resources.  For those merely reading this blog, or the others like it, I strongly recommend you set aside the time and come to a workshop.  One of the foundations for this system is that you have to experience something for yourself to truly learn it, watching or reading about it, simply don’t work.  (Yes that includes you Kahn Academy) {sorry, just had to get that in somewhere}

Modeling Unit 6

Unit VI: 2-D Particle Model

Our introductory/paradigm demo was Jon and Chris tossing a ball back and forth.  Jon then asked questions like: “Once it leaves my hand, where will it go?””Does the ball have a choice as to where it goes after it leaves my hand?”

One thing that come to mind during this demo was the following video from Veritasium.com:

What do you notice?
What can you measure?
{at this point, Jon showed us the equipment that we would be using}
{Jon build hold that converts dynamic cart w/ spring into ball launcher}
Here’s a rough sketch:

Where the blue shape is the dynamics cart with the spring plunger extended, the silver circle is the ball to be launched, and the brown shape is the holder Jon built out of wood.  He also cut/routed a groove for the ball to roll in on the top of the “shelf.”
After being shown equipment What can you manipulate?
{If you don’t have time to build this and have students tape it, use videos in loggerpro}
Open logger pro
Click Insert – Movie
Click “expand menu” in bottom right corner
Click scale icon (looks like a ruler)
Make sure you have scale (meter stick) in the movie
Click and trace standard length in screen & define length
Click on track (find name) button and click on specific point on object
Continue clicking on the same spot of the object (vernier advances to next frame)
Jon and Chris then tried to show us the classic Monkey-Blow gun demo using the Pasco equipment:

Since this is quite expensive, Jon explained how he made a “homemade version” of this:

Materials:
Electric conduit (1/2 inch? Metal)
Nail with cone of paper hot glued in
Electromagnet
Wire
12 V power source (3 or 6 V should also work)
Target – Balloon with brass mass inside, washer stretching the opening

            Stuffed animal with metal screw in its head

He attaches the Electromagnet to the ceiling in the back of his room and runs the ingoing and outgoing wires above is ceiling (drop-down I’m guessing) to the front of his room.  He uses the conduit as the blow gun and makes darts by gluing cones of paper to the head of the nail.  Have the two wires run up the side of the conduit and each extent the bare wires beyond the opening of the conduit.  Bend the wires so they touch in the middle of the opening.  As the dart shoots out, it will separate the wires, breaking the connection. Here’s his sketch:

(click to embiggen)

I missed this day of the workshop, however, a few of my cohort were gracious enough to take notes.  I’m doing my best to take what they gave me.  Any help to clarify things would be greatly appreciated.

The day began with everyone working on Unit VI worksheet.  Everyone worked individually, and then the groups met to create whiteboards.

– Useful to separate horizontal and vertical givens in table:

-Good to explicitly show + state that t is the same for horizontal and vertical motion
-Good to keep algebra in variable until the last step – then plug in number

#4 Would be interesting in adding a horizontal & vertical motion map for car and ball

-stress constant velocity in horizontal direction

– ESL students have difficulty with “how long” thinking it means distance

LoggerPro basket ball shot analysis follow up
– After students have generated data, insert 3 graphs + auto arrange

  • x vs t
  • y vs t
  • v_x vs t
  • v_y vs t

-Highlight first 1.5 second to analyze

  •  compare slope of x vs t and average value of v_x from v_x vs t graph
  • lead students to see that v_x is constant but v_y is changing: slope is 9.8 m/s^2
  • If you want, have students insert a quadratic fit onto y vs t graph and lead them to find what the meaning of the constants are in the regressed equation.

Next on the agenda was to split up an article to have summarized on whiteboards by the groups.

After lunch, Jon and Chris asked for feedback for Unit VI
What worked:
Video analysis lab
Plan for Dart Gun for Classic Monkey Problem
Worksheet #3
Wells Reading
Hammer article about Lisa & Ellen
Group Work
Adaptability of labs to every level of student (*response to comment on what didn’t work)

What didn’t work:
Transition from 1D to 2D – we would like to see the process
Time constraints
Simplicity of labs

Modeling Unit 5

Unit V: Constant Force Particle Model“Atwood Machine” with vernier track
From there, we started the next unit.  Here’s what each end of the track looked like (the middle is just a track)

Jon changed the first question slightly:
What factors will effect the motion? (between letting go of the cart and hanging mass hitting ground)

What factor effects the cart’s acceleration?
Hanging mass
Mass of car
Friction: {adjust tilt of track until cart rolls at constant speed}
(pasco hanger approx. applies force to balance friction)
Mass of pulley
Mass of the earth/gravity
Angle of the track – ?
Starting speed -?
{Chris showed us a quicker way of working through the process by guiding us to eliminate the factors mentioned that cannot be adjusted (Mass of Earth) or that could be removed with creative lab design.  The last two options we left open, that depending on your class, you may or may not want to divide and conquer.}
Purpose: What is the graphical & mathematical relationship that exist between the mass of the cart and the force that is accelerating it.
Before getting started, we talked about multiple variations to this experiment

  • Keeping the mass of the hanger while adding mass to the car
  • Using photogate(s) above the track instead of motion detector
    • Variation of this option is to attach picket fence to cart the cart and use vernier program
  • Using kinematic equations and measure total time with stopwatch for total distance measured
  • Having the students predict the mass of the system from data, and then, after showing prediction to the teacher, measuring the mass and comparing results to predictions {I like this!}
Equipment
Attach right angle to cart
Pulley at end of track
Hanger
String
Motion detector
Standard masses

Next up were some demonstrations of  Newton’s 3rd Law:
Same track set up, with Force sensors attached to the top of each car.  The twist for this demonstration is to use the magnets to apply to force between the two cars and not the direct contact.  There are a couple of things to note for this demonstration.  Have one car against the stopper and start with the second car “far away” from the first car.  Zero both probes, and make sure you reverse the direction for one of them so they both have positive in the same direction of the track.  Have the magnets inside the car so the same pole faces outward and thus repel the cars.  Push the force probe of the second car (not the car itself)
From there, were picked up on the lab with which we finished yesterday.  Before starting the experiment we briefly discussed the merit of breaking the lab groups into different types of investigations, in which we would have 3 different trials: “A” would look at keeping the mass of the cart constant, but adding mass to the hanger; “B” would keep the hanger constant, and add mass to the car; “C” would move mass from the car to the hanger, keeping the mass of the system constant.  In an effort to save time, we have everyone do option “C” but I may or may not look at all the groups (maybe in my honors class?)
Also showing a way to move through the whiteboard process more quickly (as needed by time constraints or if class is not productive in meetings), Jon walked us through a “Circle the wagons” meeting.  In this format, all the groups show their white boards, and the teacher leads the group to try to draw conclusions in looking at all the results at once.
{As we were getting started, Jon also mentioned that when you are “normal” whiteboard meeting after a lab, in subsequent labs, start with a different group each time and change the order you call the groups forward.}
During the meeting, we had a great discussion on whether you should explain/guide to the students before starting the lab that they will need to plot Force vs acceleration so that the slope is mass, or wait until the end.
{My thought is to wait until the end, have all the groups manipulate their graphs, as teacher does it on projected screen}
At this point, Jon showed us a quick follow up demo/lab (used vernier “Lab 9 – Newton’s 2nd Law”)
Jon taped an accelerometer to the force probe (Jon uses Velcro tape at his school).  Then you just click the record data button, and then push the cart back and forth.  Viola, data showing $F \propto a$
From there, we started individual work on Unit V worksheet 1 (#’s 1-4) and worksheet 2 (#’s 1-3)
As we got started, we briefly discussed strategies for word problems (w/ forces).  A summary of what we said was:
  • Have students sketch what is happening and identify the system with dotted circle/box
    • Get the words out of the word problem
  • Create a Free Body Diagram
  • Next to FBD, draw an arrow showing the direction of acceleration
    • That will be “+” direction for the problem
      • This convention will aid circular motion problems later in the year
Notes from whiteboard session:
  •  Wkst 2: #2 is a great problem since a given number isn’t use in the calculation, but rather for analysis at the end.
  • Wkst 2: #3 mass not given, so students need to determine it from the Weight
    • Chris- Make sure units are included in the calculation not just at the end
    • Possibly change wording of problem since the normal force changes not F­­­w
 Jon then went on to describe how he helps his students understand “elevator” problems.  If you are standing on a bathroom scale, and you want to increase your “weight” you can pull on the bottom of the counter and squeeze the scale.  This is the same effect as when the elevator is accelerating upwards.  On the contrary, if you want to lose “weight,” you can push on the top of the counter and push you body off the scale.  This same effect occurs when the elevator accelerates downward.
From there, we Jon showed us some fun demos
  1. Have student kneel w/elbows touching knees & hands “praying”.  Put chapstick at tip if fingers.  Then have student place hands behind his/her back. They then need to try to knock over the chapstick by touching their nose to it.  Due to differences in center of mass, girls should be able to do this, while boys usually can’t.
  2. Have student stand facing the wall, with toes touching the base of the wall.  Have student take 3 steps (toe to back of heel) away from the wall.  Bend at the waist $90^o$ with their forehead touching the wall.  Place a small chair (or other “small” mass) in their hands and tell them to stand up.  Again, boys will struggle,  girls will tend to be successful.
  3. Have one student (biggest student) sit all the way back into a chair with his/her feet flat on the floor.  Have a second  student (smallest) stand in front of first student and push into the first student’s forehead.  Tell first student, without moving their feet, to stand up.  At the same time, the second student pushes on the forehead of the first, preventing him/her from standing up.
  4. Have student stand with right shoulder and outside of right foot touching the wall.  Then tell the student to lift his/her left foot.
Friction Lab
We then moved on to a lab on Friction. We used a friction block and force probe.  The basic procedure was to the block at constant speed with different masses resting on top of the block.  We used the vernier file “Lab 12a Static Kinetic Fric.”  A sketch of the graph produced looked basically like this:
The max force represents the static friction force, and when the force is basically horizontal (red line) then the friction force equals the measured force.



To speed things up, each group given different normal force (“zero mass” was mass of block plus 250 g) and needed to get good data (slope of oscillating data was as close to horizontal as possible).  Find the average value of the force using the statistics button.
Unit V Feedback
The Good:
  • We felt we were becoming comfortable using computer based equipment
  • Continuation of the sequence of showing 3rd law  (adding non-contact interaction)
  • Low tech demo’s/labs
The Bad:
  • Modified atwood machine has a lot of physics baggage we’ll need to use as a paradigm

Suggestions:

  • When it comes to multiple representations of event
    • FBD, motion maps, graphs, equations
    • Only let students say verbal description of the event– Carbon dioxide not “See” “Oh” “Two”