Ball in water on elevator: Buoyancy

OK. Float a ball in a beaker of water in an elevator. The ball floats because the buoyant force of the water displaced by the ball is equal to the weight of the water.

Now, accelerate the elevator upward. What happens to the ball? Does it: Sink lower, rise higher, or stay the same?

Here’s my video with the answer:

Once the video is playing, you can click on it to get to the YouTube page with larger size and high definition versions.

Force data from elevator ride to top of 555 California St, SF (52 floors)

force plate and LabQuest
Graph of change in acceleration in “g’s”

Yesterday I took a trip up the longest elevator ride in San Francisco (at 555 California St). I brought along my LabQuest and Force Plate (essentially a recording bathroom scale, see image of LabQuest at bottom of post). As I rode the elevator up and down, the LabQuest was recording my apparent weight (think about how you feel when an elevator first starts moving up or down, your weight seems to change).

I made five round trips, three recording my own weight, then two recording the weight of my Timbuk2 bag (I noticed I was bouncing quite a bit on the ride, and thought my bag might bounce less).

If you have Vernier LoggerPro software (worth the price for their video analysis even if you don’t have any of their sensors), you can download and work with the file. Here it is:


I did some data processing in this file; feel free to use it or delete it if you want your students to have to do it all themselves. The graph above was created by dividing the force on the scale by the initial (resting) force, then subtracting one to the the relative change in apparent weight. The maximum and minimum are both about 0.12g, or about 12% of gravity.

force plate and LabQuest
Force plate and Labquest

One discovery I found interesting is that the elevator does not have a continuous acceleration as it gets to its maximum speed, but rather starts with a high acceleration, then decreases its acceleration until it gets to its highest speed. When slowing down, it does the reverse: It starts with a low acceleration, increasing it as it slows until it comes to a stop and stops its acceleration (non-physics geeks: in physics, we don’t use the word “deceleration,” but rather just use positive and negative to tell direction).

I rode up the “express” elevator to the restaurant on the top. The security manager pointed me in that direction once I told him I was just wanting to collect the data for my classroom, not that I wanted to bring a bunch of high school students up with me. The express elevator turned out to be the best option, since I was able to see the maximum acceleration without having to stop at intermediate floors.

Tips for using the TI 83/84 calculator in a science class

I’ve posted my student handout “Analyzing data using your TI-83 or TI-84 calculator” to the web. You can find it and more TI tools at

[October 23, 2009: I have an updated version you can find here.]

I presented this handout at the Northern California/Nevada American Association of Phyiscs Teachers November 8 meeting at Foothill College.

Comet 17/P Holmes starting to light up the sky

There’s a comet that’s starting to sparkle the night sky: Comet 17/P Holmes. It is said to look quite nice with binoculars or a small telescope, but is also visible with the naked eye.

NASA has a great 2D model of the comet’s orbit. (You’ll need Java, but almost all computers these days have it installed.)

I’ll post more soon, but if you have Stellarium (a great open-source planetarium program), you can add it to your ssystem.ini file by entering the following information:

name = 17P/Holmes
parent = Sun
radius = 1200
oblateness = 0.0
albedo = 1.0
lighting = true
sidereal_period = 2513.00981
halo = true
color = 1.0,1.0,1.0
tex_halo = star16x16.png
tex_map = nomap.png
coord_func = comet_orbit
orbit_Epoch = 2454221.5
orbit_MeanAnomaly = 359.49863
orbit_SemiMajorAxis = 3.6174053
orbit_Eccentricity = 0.4324195
orbit_ArgOfPericenter = 24.25862
orbit_AscendingNode = 326.86739
orbit_Inclination = 19.11331

Thanks to the great posters at SourceForge for this information.

Once you enter it in your program, you can see exactly when and where Comet 17/P Holmes will be visible at your location.

Another day, another whiteboard

White board of pendulum labToday we did a lab where we examined the variables in the swinging of a pendulum. We changed the length of the string, the mass of the pendulum, and the angle it was released from. We recorded the period (time) for each trial. I won’t spoil it and give you the results, so you can try it yourself if you want. We learned more good ways to do this introductory lab. My group’s white board is to the right.

When we finished with the lab, the next part was looking at ways of introducing non-linear graphs to students. The basic technique is students are provided with non-linear equations (y=k/x, y=kx^2, y=kx^(.5), etc.) and asked to graph them. After graphing them, they are asked to explain what is the slope and y-intercept mean in each.

After this, students are given non-linear data that they graph and go “ack, I can’t make a linear equation (y=mx+b) from this.” They are then asked to look at the graphs they just made and determine if their new graph looks like one of them. They match it to the closest graph, then take the independent variables in their data and process them according to the equation of the matched graph (e.g. square, take the square root, take the inverse). They then graph that data and tada it forms a nice linear graph. They can then write the equation for this new line.

So what is “Physics Modeling” anyway

One whiteboard with c over f

The bulk of my trip this summer is being spent in a training in the Modeling Instruction in Physics technique. The term Modeling comes from the perspective that physics is a set of models of the physical world. These models can be mathematic (equations), diagrams, and literal. More details from their web site:

The Modeling Method of High School Physics Instruction has been under development at Arizona State University since 1990… The program cultivates physics teachers as school experts on effective use of guided inquiry in science teaching… Program goals are fully aligned with National Science Education Standards. The Modeling Method corrects many weaknesses of the traditional lecture-demonstration method, including fragmentation of knowledge, student passivity, and persistence of naive beliefs about the physical world. Unlike the traditional approach, in which students wade through an endless stream of seemingly unrelated topics, the Modeling Method organizes the course around a small number of scientific models, thus making the course coherent.

I think I’m pretty good at integrating inquiry based student centered curriculum into my classroom, but I have not managed to infuse it throughout. I’ve been wanting to attend a Modeling workshop for years, and this year I finally opted to do it. Workshops are offered all over the country each summer, and the ones I could fit into my schedule were in NC or Arizona. Since I have more friends and family in NC than Arizona (Karen and Chris, I still love ya), I decided on NC.

The course I’m in has only six participants, which is a little disappointing, but not that many teachers are willing to put three weeks of their summers into a specific training program. The design of the training is that we (the teachers in the training) work most of the lab activities ourselves, reviewing them at the end to reflect on the goals of the activity. Our first activity today was to examine the nature of a ball bouncing. The basic design of the activity was:

  1. Teacher demonstrates a bouncing ball and ask what we observe.
  2. A list of observations are recorded (e.g. it doesn’t bounce back as it fell, it makes a noise when it hits the ground).
  3. The teacher leads the class in a discussion of which of these observations lead us to items we could measure.
  4. The class settles on comparing the height of the drop to the height of the bounce.
  5. Lab groups meet and develop their technique for a lab that will compare this (no specific instructions are given).
  6. Lab groups conduct their experiment.
  7. My first whitboardLab groups write a “white board” with their data, analysis, and conclusions (the white boards are about 2’x3′, see my partner’s and my whiteboard on the right — click on it for a larger version that you can actually read 🙂 ).
  8. One group presents their lab to the class (using the white board to show their “report”), with the teacher and students asking questions as needed.
  9. Other groups present their labs. As the new reports are made, the teacher helps to guide the discussion to similarities and differences in the reports (both results and design).
  10. After all the presentations are made, a class discussion is held where the nature of the relationship between the variables (drop and bounce heights) is examined.

Notice there’s no teacher telling students “this is how it works.” Research in education has demonstrated the importance of students examining their own beliefs about a subject before learning new material about the subject. When students have misconceptions about a topic that is not addressed before learning, the students tend to place the new material in their short term memory, then quickly revert to their previously held misconceptions about that topic. If, on the other hand, they address their beliefs before the topic is presented, they better integrate the new and old, notice the differences, and, when the new knowledge is integrated into their minds, they are better at dismissing their old misconceptions because they see how they are wrong.

Well, enough for today. The class was great, and the two instructors seem quite good at teaching the class. I’m looking forward to tomorrow and the rest of the three weeks.

Signing off from Charlotte,