Today we finished up the paradigm lab for centripetal acceleration, and then did a goal-leas problem using the typical spinning stopper lab. These samples of the work students were doing. My favorite is the one on the vertical whiteboard, where the student drew in Fm for “mysterious force,” since he knew something had to balance gravity but didn’t know what it was.
The data for acceleration vs. speed for the spinning chairs was not super convincing in either of my classes, as far as trend goes. Linearizing to a quadratic didn’t ‘look’ much better than a simple linear function. Then I thought about intercepts.
The non-linearized intercepts were all something significantly non-zero; but if the intercept was non-zero, that would mean that we had an acceleration when the chair was at rest. By contrast, the linearized graphs had intercepts that made sense, very close to zero. This plus the fact that they did generally fit a bit better convinced students, I think, that acceleration is proportional to the square of speed. We then talked about how when you speed up just 10 mph going around a corner, the effect you feel seems to increase significantly, once again indicating something more than a linear trend.
We ended the day talking about how the direction of the force on an object is related to the direction of the acceleration, so we tried to make bowling balls move in circles.
Tomorrow we will investigate the effect of radius on acceleration, and we should be able to pull everything together on Wednesday.
For the next couple of days we will be investigating the factors that affect centripetal acceleration. On Friday we took data to look at the relationship between the speed of the object (the Labquest) and the acceleration. We’ll be graphing and white boarding on Monday, I’m excited to see how the results pan out!
No, I didn’t skip a day….I have to write Friday’s post still, but wanted to do this while it was fresh on my mind. Both Frank Noschese and Kelly O’Shea (in conversation and probably in writing but I can’t find it) have talked about having students check their work and give themselves feedback directly after taking a quiz. I gave my first Standards-Based Grading quiz on Friday (Yay! I’m teaching the 2nd half of physics this semester which I helped change to SBG for the start of the year despite not teaching it), and immediately when students finished they could go to the back, grab a red pen, and give themselves feedback. The quiz above is a great example of a student where I had much less feedback to give (mine is in blue) because she did it herself, plus she got that information when it was fresh in her mind rather than after a weekend. Win-win!
Today we started by figuring out how an accelerometer in a labquest works, then confirming the directions and signs for the x,y, and z axes. Then I had students predict what the x, y, and z acceleration graphs would look like, and we quickly shared our thoughts. Many groups had something like below;
One class had a particularly interesting discussion about how they would expect the graphs to be sinusoidal if the labquest were maintaining a constant orientation (ie, if the x axis was always pointing west, rather than our method which was to have the x axis always pointing tangential to the circle, so it changes direction in an Earth reference frame) vs our method.
In the end, we spun, and we got some pretty decent data showing the y axis had negative acceleration (when the screen was pointing up), showing the acceleration was toward the center of the circle. They also picked up quickly that the speed of rotation seemed to have an effect; we will investigate that tomorrow.
This semester, which started yesterday, I inherited a section of ‘regular’ physics, which I did not teach last semester; all sections were taught by a colleague and friend. I had my first whiteboarding session today, and it was awesome. Presentations were great, kids made mistakes and were totally ok with that (that’s a good thing), and the audience participated both by asking questions and by modifying their work on their worksheets. Thanks to Ben for setting me up with kids who already know how to whiteboard!
I wrote previously about a new demonstration I was planning to aid in teaching relative velocities. I would love to have students do it, but the setup, including the camera in the ceiling, was a bit too intense once I did a quick cost/benefit analysis of the gain for that vs. doing it as an interactive demo. I think it really helped in their understanding of how velocities can add, but we’ll see when we whiteboard some problems tomorrow.
We looked at some plane landings too. It’s pretty fun pausing it before the first one touches down and forcing them to draw the vector addition diagram before letting them see the result!