# N3L Activity Stations

While the [Newton’s 1st Law activities](https://pedagoguepadawan.net/147/n1lactivitystations/) serve as a fun and short introduction, the Newton’s 3rd Law activities provide a shared experience that spans several classes. The activities that the students explore are selected to highlight the most common preconceptions that students have about Newton’s 3rd Law. I stress how important free-body diagrams are as a tool in their physics toolbox and that, once they are adept at drawing free-body diagrams and once they actually trust their free-body diagrams, they will be able to explain a number of counter-intuitive situations. I introduce these activities by stating that Newton’s 3rd Law is one of the most easily recited laws of physics and yet is least understood. Here are the activities:

**Sequential Spring Scales**

The spring scales are initially hidden under the coffee filters. Only after students make their prediction are the coffee filters removed. Most students do not predict that the spring scales will read 10 N. Some predict 5 N (the spring scales split the weight). Some predict 20 N (10 N each way adds up to 20 N). In addition to drawing the free-body diagrams, this scenario can be explored further by asking students to predict the reading on the scales if one of the weights is removed and the string is tied to a clamp instead.

**Bathroom Scale**

This station provides an important shared experience that we will refer back to when discussing the elevator problems later in the unit. This station also generates a number of excellent questions such as “would the scale work on the moon?” and “how could you measure mass on unknown planet?”

**Twist on Tug-of-War**

Students were very interested in this station this year since they were in the midst of Homecoming Week and inter-class tug-of-war competitions were being held. It may have been the first time free-body diagrams were used in the planning of the tug-of-war team’s strategy. The dynamics platform in the photo is a cart build from plywood and 2x4s with rollerblade wheels and has little friction. Most students claim that whoever wins the tug-of-war pulls harder on the rope than the person who loses. Only after drawing the free-body-digram and trusting it, do they realize this is not the case.

**Medicine Ball Propulsion**

This is a fairly straight-forward station. I often wander by and ask the students exploring it why they don’t move backwards when playing catch under normal situations. I also check at this point and see if they are convinced that the force on the ball by them is equal to the force on them by the ball.

**Computerized Force Comparison**

*This is the most important station in that it helps students truly appreciate Newton’s Third Law.* I setup several of these stations to make sure that everyone has an opportunity to watch the graph in real-time as they pull on the force sensors. This is the standard Modeling activity for Newton’s 3rd Law. For students still struggling to accept Newton’s 3rd Law while working through this activity, I challenge them to find a way to pull on the two sensors such that the forces are not equal in magnitude and opposite in direction. This activity also counters the misconception promoted by some textbooks (perhaps unintentionally) that the “reaction” force follows the “action” force. Students can clearly see that both forces occur at the same time. (We refer to paired forces according to Newton’s 3rd Law, not action-reaction forces.)

**WALL-E and the Fire Extinguisher**

Who doesn’t love WALL-E? I repeatedly loop through a clip from the [WALL-E trailer](http://youtu.be/ZisWjdjs-gM?t=2m26s). In addition to the questions on the handout, I ask students what is incorrect about the physics in the scene. This year, I also showed students this clip that [Physics Club](http://physicsclub.nnscience.net/) filmed several weeks ago:

# N1L Activity Stations

I like to introduce Newton’s First Law with a series of activity stations for students to explore followed by a couple of demos. They have fun and it provides shared experiences which we can refer back to later. Here is the activity sheet that guides them:

Many of these stations and demos have as much to do with impulse as they do with Newton’s First Law. I mention this and we revisit these stations and demos later when studying impulse.

Most of these stations and demos are fairly self explanatory. However, a few can benefit from a photo. Here is the “Nuts about Hoops & Bottles” station:

You quickly grab the hoop with a fast, horizontal motion. This station can become overcrowded because some students obsess over trying to capture the most nuts in the bottle. (I’ve seen students catch over twenty.)

The “Hitting the Stake” station is perhaps the most surprising to students. It is easy to build and looks like this:

The “Spin the Human” station works best on teachers with little hair. We have one constructed from pool balls. This one is built with golf balls and a coat hanger:

It is best to put the “Chopping Blocks” station in the corner. Some students have an incredible amount of aggression to release.

I’m sure everyone has seen the “Clearing the Table” demo. If not, MythBusters has an [extreme version](http://dsc.discovery.com/videos/mythbusters-tablecloth-pull-high-speed-2.html).

A couple of years ago, I captured the “Egg Drop Soup” demo with the high-speed camera. I usually have all four eggs make it.

What is interesting about these activities is the evolution of this lesson. When I started teaching, these were all demos. I put on the show and the students’ engagement was that they laughed. A few years ago, my team transitioned these from demos to activities. More fun, more engaging. Based on a suggestion from my instructional coordinator, I now introduce each station and have the students record their predictions before get up and start visiting stations. This ensures they actually make predictions since many of these stations are too enticing for them to make predictions before playing with them.

Maybe I’ll let students “Clear the Table” next year.

# CV Buggy Lab

Last week, I participated in a great discussion on Twitter about the various ways Modelers perform the Constant-Velocity Buggy Lab in their classrooms. The CV Buggy Lab is the paradigm lab for constant-velocity and, as a result, Modeling classrooms are filled with toy cars in the fall. I’m not sure why, but it seems that the red cars are always configured to go “fast” and the blue cars configured to go “slow”.1

We’ve always done a CV buggy lab, even before I started modeling, but this year we did something different. To provide some context, before we do the CV buggy lab, students have already completed a mini-modeling cycle involving the bouncing ball and explored non-linear relationships with the sliding box of mass and rubber bands. We have also briefly discussed the concept of position in terms of specifying the location of something relative to a commonly defined point. For example, “my chair is 5 floor tiles from the south wall and 10 floor tiles from the west wall.” Another teacher and I were discussing that since students were rocking these labs, our typical buggy lab that involves only one car might not be as engaging or beneficial. She decided to have students start with both cars from the start. I thought this was a great idea and decided that I also wanted each group to analyze a different scenario which would make the post-lab whiteboards discussion more interesting.

As a class, we go through the usual process of making observations, determining what we can measure, and, eventually, coming up with the purpose for the lab:

To graphically and mathematically model the relationship between position and time for two buggies traveling at different speeds.

At this point, I had to constrain the lab more than I usually would by specifying the starting position and direction for each car. I assigned each lab group a scenario (this allowed some degree of differentiation in terms of difficulty):

1. red positive direction, blue negative direction; red at 0 m, blue at 2 m
2. red positive direction, blue negative direction; red at -1 m, blue at 1 m
3. red negative direction, blue positive direction; red at 2 m, blue at 0 m
4. red positive direction, blue positive direction; red at 0 m, blue at 0.5 m
5. red positive direction, blue positive direction; red at -1 m, blue at -0.5 m
6. red negative direction, blue negative direction; red at 2 m, blue at 1.5 m

Their homework was to draw a picture of their scenario and brainstorm on how they would design the experiment.

The next day, groups designed their experiment. I didn’t provide any additional restrictions. I only verified that their pictures matched the scenarios that I had specified. Some groups decided that their independent variable would be time; others, position; others, distance. One group decided to gather data from both cars at the same time! Another group taped a marker to the back of the cars which traced their paths on butcher paper and allowed them to make more accurate measurements of the actual distance traveled.

When groups started graphing their data, I requested that they plot time on the horizontal axis. Some objected and remarked that if time was their dependent variable it should be plotted on the vertical axis. I explained that I wanted all the groups to be able to share their results which would be easier if we used a common set of axes. I reassured them that the graph police would not come and get them for plotting their dependent variable on the horizontal axis. (Anyone know why this is the convention?)

Some expected and unexpected issues emerged as students began to graph their data. As expected, those groups who chose to measure distance instead of position soon realized that their graph wasn’t going to convey everything they wanted. They went back, and using their picture, calculated positions corresponding to each distance. We use LoggerPro for graphing, and those groups who made time their independent variable, simply added a new column for the position of the second buggy. LoggerPro makes it super simple to graph multiple sets of values on the vertical axis (click on the vertical axis label and choose Moreâ€¦). However, those groups that made position their independent variable had more trouble since LoggerPro only allows one column to be plotted on the horizontal axis. These groups required more assistance and, in the end, I discovered that it was best to create two data sets and name the time columns identically for each. LoggerPro would then plot this “common” time column on the horizontal axis and the two position columns on the vertical axis. Not super simple, but doable.

Each group drew their picture, graph, and equations on a whiteboard. We did a “circle whiteboard” discussion rather than having each group formally present their results. At first, the discussion focused on how the graph described the motion of the buggies. As students became more comfortable with those ideas, the discussion shifted to comparing and contrasting the different whiteboards. This was the best whiteboard discussion for the CV Buggy Lab that I have ever had. At the end of class, I confidently shared that their whiteboards captured everything that we would learn about constant velocity. We just needed more time to digest, appreciate, and refine what they had already created.

I’ll definitely do this again next year, but I hope to find a way to not assign each group a scenario and yet still end up with a variety of initial positions, directions, and relative motion. Perhaps, if I ask each group to design their own scenario, I can subtly encourage small changes to ensure the variety still exists. Plus, students usually create scenarios that I never would consider!

1 There are many ways to make the blue buggy slow. I have used wooden dowels wrapped in aluminum foil and wooden dowels with thumbtacks and wire. Others have shared that they use dead batteries, electrical tape, and aluminum foil. This year, I tried something completely different. I found these wires with magnetic ends while cleaning last spring (I have no idea who sells them). While in previous years, it seems that in every class someone’s blue buggy has an intermittent connection, I had no problems at all this year.

# Measurement Uncertainty Activities

I was inspired after a recent [Global Physics Department Meeting](http://globalphysicsdept.posterous.com/#!/), where we discussed uncertainty, to update the measurement uncertainty activities we do at the start of the year.

I just finished these activities with my Honors Physics classes.

I have a different purpose in mind for each station beside practicing measuring and the crank-three-times method (I found [this document](http://www.av8n.com/physics/uncertainty.htm) extremely helpful in refining my understanding of uncertainty and introducing me to the crank-three-times method):

1. **area of the desk**: I want students to appreciate that using a reasonable measuring device can result in results with relatively small uncertainties. I also wanted students to appreciate how the uncertainty of individual measurements are compounded during calculations. I was pleased that students mentioned how the curved edge of the desk made this measurement more uncertain and how ensuring that the meter stick was parallel to the side being measured was challenging.

2. **classroom volume**: I want student to appreciate that the uncertainty of a measurement is not solely due to the measurement device (e.g., the meter stick) but also to how you use it (e.g., having to lay meter sticks end-to-end or marking and moving a meter stick). This is also a good opportunity for students to learn to express results using unit prefixes that are easier to comprehend. Cubic meters work better than cubic centimeters.

3. **dime volume**: I want students to appreciate that what is a reasonable measuring device for one measurement is not for another. You shouldn’t use a ruler to measure the thickness of a dime; if you do, your uncertainty as a percentage of your measurement is huge. Students suggested using both alternative measuring devices (e.g., calipers) as well as entirely different techniques (e.g., water displacement of multiple dimes).

4. **time light**: I wrote a LabVIEW VI that lights a bulb on the computer screen for a specific amount of time. This activity reinforces the lesson from #2 (i.e., the uncertainty of measuring a time interval with a stopwatch is overwhelmingly due to human reaction time and not the precision of the stopwatch display). I also wanted to gather this data to calculate the uncertainty of this type of measurement which we can use in future labs. Below are the results.

5. **cart on a ramp**: This also reinforces the lesson from #2 but involves additional uncertainty due to the interaction of multiple people (i.e., one person calling out second intervals and others marking position). Students realized that they couldn’t define a single measurement uncertainty for all position measurements since it appeared that the uncertainty was greater the faster the cart was moving. I also wanted to gather this data to calculate the uncertainty of this type of measurement which we can use in a lab next week.

6. **pendulum period**: I want students to realize that the experimental procedure can have a dramatic affect on uncertainty (i.e., timing 10 cycles results in much less uncertainty than timing just one).

Throughout the day, we captured 275 time measurements for the blinking light. I created a histogram in LoggerPro and calculated the standard deviation:

The distribution appears to be gaussian in nature and the standard deviation is 0.1 seconds. So, this year, when using a stopwatch to measure a time interval, we will use Â± 0.1 seconds as our measurement uncertainty. The actual value programmed was 4.321 s.

Here are the histograms for the position measurements:

The distributions for the position measurements had much greater uncertainty than I hoped. Also, they were more complicated to make; so, I don’t have as much data as I do for the timed light. I’ll have more classes do this activity next week which will provide more data. Regardless, we may need to reconsider next week’s accelerated motion lab since measuring position visually based on a stopwatch time has a very high uncertainty. In past years, we used spark timers and tapes for accelerating objects, but our spark timers no longer make clear dots on the tape. Any suggestions?

# Holography

There is a long tradition at my school of students creating holograms as a final activity in physics. Everyone gets to make their own and keep it. I have heard several alumni mention that they still have their hologram. Just this week, an alumni who is also a dean remarked that he still has his hologram from 20 years ago. Sometimes the purpose of an activity is learning; sometimes, just to inspire. This is the later.

I’m not sure how we first made holograms, but at some point in the distant past, now retired teachers must have attended a holography workshop led by Dr. Jeong from Lake Forest College. A couple of summers ago, I attended a Chicago Section AAPT meeting and was surprised to learn that Dr. Jeong was leading the workshop!

For years we have been making [reflection holograms](http://www.integraf.com/a-simple_holography.htm). These usually turn out well. The disadvantage is that there isn’t much depth and, therefore, the 3-D effect isn’t as dramatic. The advantage is that reflection holograms are easily visible in white light (sun light is especially effective).

Last year, after attending Dr. Jeong’s workshop, we decided to try and also make [transmission holograms](http://www.integraf.com/a-make_transmission_hologram.htm). Dr. Jeong actually demonstrated how to make an “omnigram” which is a combination of a reflection hologram and a transmission hologram on a single slide. We tried this, but only the transmission holograms were visible. The transmission holograms were amazing. They have an incredible depth which allows larger objects (or a collection of small objects) to be captured. The disadvantage is that a laser is required to view the hologram (a green laser pointer works better than a red one).

This year, we provided students an option to make either type of hologram. They split about 50-50. As the price of laser pointers continue to fall, we may soon only make transmission holograms.

We order all of our supplies from [Integraf](http://www.integraf.com/). We use the PFG-03M holography slides, the JD-4 processing kit, and the DL-4B laser diode. The setup for transmission holograms is relatively simple. I have detailed photos of the slide holder (on the left) and laser (on the right). The objects are positioned between the slide holder and laser. In the back, is the shutter which blocks the laser light and consists of foam board covered with black felt with a base of two large binder clips.

I built the slide holder from a 2.5″ picture frame. The picture frame is painted a flat black. Black backing material is glued to the top of the picture frame to ensure that laser light does not enter the sides of the slide. The picture frame is secured to a base which is a tea tin filled with sand and covered with black felt. The picture frame backing is slid behind the slide in the frame to secure the slide (emulsion side faces the scene).

The diode laser is secured by a clothespin in a tea cup filled with sand. It is is positioned on a base which consists of three physics texts covered with black fabric. I added a switch and a two-pin connector to the battery box which results in a more reliable connection and easier operation.

If you are interested in making your own holograms, feel free to contact me and I’ll try to answer any questions that you may have. Dr. Jeong is very approachable and provided several tips based on questions that I posed.

**Update**

I realized that it would be helpful to show some examples of these holograms. It was challenging to photograph them, but here is my best attempt for a transmission hologram:

And here is a reflection hologram:

# Circuits Lab Practicum

This year, we created a new lab practicum for the circuits unit. In addition to the traditional activities of having students draw a circuit diagram from a written description, build the circuit, and measure the voltage across and current through a specified resistor; students had to infer the circuit diagram for a collection of lightbulbs based on their observations.

This activity was inspired by an old Science Olympiad circuits event. As shown in the following photo (which is somewhat hard to discern due to the pattern of the fabric), four labeled light bulbs protrude through holes in the fabric. The fabric hides the wires connecting these light bulbs. Students turn on the power and then make observations by unscrewing and screwing in the light bulbs. Based on their observations, they draw the circuit diagram and justify their conclusion.

Students were most engaged in this activity of the lab practicum compared to the others. I think the fact that it was a unique way for them to apply their knowledge and inference abilities made it so interesting. It also had the unexpected benefit of reinforcing the idea that physical order of the light bulbs has no effect on their brightness. That is, the first light bulb in series from the positive terminal of a battery is not the brightest because it is “first.” Several students commented that there were several circuit diagrams that they could draw that would match their observations. It was reassuring that they came to this conclusion!

# Measurement Uncertainty Activity

In previous years, my students have always struggled to really understand measurement uncertainty. Due to my background in the computer-based measurement and automation industry, I was always troubled that I didn’t do I better job helping them understand. So, this year, I developed a set of six activities to provide a hands-on way to practice applying the definitions as well as provide a context to discuss the complexities of measurement uncertainty. Each group investigated one of the activities and then whiteboarded and presented their results with the rest of the class. Each activity had the group determine the measurement uncertainty of a measuring device and calculate the maximum percent uncertainty of their measurements. However, each activity also had a deeper purpose that led to good class discussions during whiteboarding.

1. Measure the dimensions of a block with a ruler. Deeper purpose: calculate the percent uncertainty of the volume of the block.

2. Measure the width and length of the lab table with a modified meter stick (cm precision). Deeper purpose: how does having to make multiple measurements to measure the length affect the measurement uncertainty?

3. Measure the period of a pendulum with the wall clock. Deeper purpose: how does the percent uncertainty change if 2, 5, 10, or 20 oscillations of the pendulum are measured instead?

4. Measure the temperature of ice water and hot water with a digital temperature probe. Deeper purpose: is the percent uncertainty of the cold-water measurement actually greater than that of the hot-water measurement? How does measuring the temperature differ than all the other measurements (difference vs. absolute)?

5. Measure the time for a ball to drop from the table to floor and the ceiling to floor with a digital stopwatch. Deeper purpose: Are the measurement as precise as the measurement uncertainty of the digital stopwatch (1/100 of a second)?

6. Measure the speed of the cart on the track using a photogate connected to the computer. Deeper purpose: What does the computer actually measure? What determines the measurement uncertainty? Determining the actual uncertainty of a photogate connected to a laptop running Logger Pro via a LabPro is well beyond the scope of this course (although, in my Advanced Physics course, we figure it out). Still, students realizing that computer-based measurements don’t have infinite precision is an important lesson.

The class discussions that occurred while whiteboarding were fantastic and this year’s students have a much greater appreciation of measurement uncertainty than those of previous years.

# Polar Bears around an Ice Hole

I started some of my classes today with the “Polar Bears around an Ice Hole” riddle:

> The game is in the name of the game â€“ polar bears around an ice hole â€“ invented in the days of Ghengis Khan.
>
> A clue for you to keep you true â€“ like petals around a rose, you can count each bear’s nose.
>
> How many polar bears do you see?

You then roll a bunch of dice. (I created six 5″ dice from styrofoam and black pom poms.) A physics teacher from another school uses this as his introduction activity on the first day of school and shared the activity a couple of years ago. I had planned on using this activity as an extended analogy to introduce specific aspects of the class culture:

* You may feel frustrated as you try to figure physics out. That’s okay.
* Physics is hard to understand until you know the “rules of the game.”
* But, once you discover the rules, physics often seems easy and you may be surprised that others don’t understand.
* However, remember that you didn’t always understand.
* When you discover the rules and understand without someone just telling you the “answer”, you are excited.
* The journey to understanding is very important. So, no one is going to tell you the answer, but we’re all here to support each other on our journeys.
* Being told the “answer” at most gives you one answer that you didn’t know. Learning to think critically and arrive at the answer with support develops a skill that you will use to find many answers.

As the activity progressed, I realized that this activity also served as an excellent example of scientific inquiry. As we continued to try and solve the riddle, I introduced several important ideas:

* make careful observations
* gather lots of data (many roles of the dice)
* look for patterns, compare and contrast, look for extremes
* simply the problem being investigated (roll fewer dice)
* constrain the variables (set dice to specific values)
* propose a hypothesis, test it, modify it based on results, repeat

After discussing the activity, I grabbed my notebook and nonchalantly asked who solved the riddle within the first five minutes. I then announced that they would receive As for today. I then asked who solved the riddle in ten minutes and announced that they would receive Bs. Next, who solved the riddle in fifteen minutes and announced that they would receive Cs. Everyone else would receive Fs. This provided a great hook to transition to our discussion about standards-based grading.