College-Prep Physics: This actually happened a few days ago, but it’s too awesome not to share.
In an attempt to differentiate acceleration practice, I gave students a few choices. They could work on practice problems, work on graph interpretations, or do an acceleration challenge. Only one group of students from all 3 of my sections decided to go with the challenge. The goal of the challenge is to determine how far up the ramp to release a free-rolling cart so that it collides with a constant speed buggy at the bottom of the ramp. Students were not allowed to attempt any collisions, but could take data in order to determine the speed of the buggy and the acceleration of the cart down the ramp.
There are several ways to determine the cart’s acceleration:
- Collect position-time data by hand and calculate acceleration using kinematics.
- Film it and use Logger Pro or Tracker video analysis
- Use a motion detector and get the slope of the velocity-time graph
This time, the group decided to try the Ubersense app to take video and use the frame-by-frame feature to extract position-time data rather than using a stopwatch. They marked off 1 meter distances and videoed the cart rolling down the ramp. No need for the whole trip to be visible within the entire video frame — we can follow along with the cart and use the frame-by-frame playback to extract position-time data from the video:
Then they input the data into Desmos and tried to find a parabola of best fit to model the cart’s position as a function of time.
Next, they found the speed of the constant velocity buggy. Then I told them how far away from the crash point (X) the buggy would start. They then calculated the time it would take the buggy to travel to the X. Then they used that time and the Desmos graph to determine the distance the cart would travel down the ramp in the same time (e.g., the starting position of the cart).
As you can see at the beginning of the post, they nailed it!
AP Physics C: If you stick a piece of invisible tape down on table and pull it up quickly, it becomes charged. How much charge is on the tape? What percentage of the atoms on the surface of the tape give up (or gain) an electron? That’s what we were figuring out today. There are several methods. Pictured above is what I call the “angle method.” The group below is levitating the top tape via repulsion from the tape underneath.
While the following group is levitating the bottom tape via attraction.
AP Physics C: Students were tasked with writing a program in VPython or GlowScript to model the electric field around a dipole. Here’s my GlowScript program. (In GlowScript programs, rotate the camera by dragging with the right mouse button, or hold down the Ctrl key and drag. To zoom, drag with the left+right mouse buttons, or hold down the Alt key and drag, or use the mouse wheel.)
College-Prep Physics: What if there’s more than one force? We used dueling fan carts and a motion detector to predict and test the outcomes of several scenarios. It turns out that …
2 carts, 1 fan on high (3F) ≈ 0.3 m/s/s
2 carts, 1 fan on medium (2F) ≈0.2 m/s/s
2 carts, 1 fan on low (1F) ≈0.1 m/s/s
So, what happens in the balanced cases?
Not terrible surprising. How about the unbalanced cases?
Then we returned to the PhET force and motion sim. We pushed the sleepy dog with 200 N (F1), turned friction on/off (F2), and looked at the resulting acceleratations:
Then I challenged the students to determine:
- When there’s friction, how hard should we push so that we get the same acceleration as without friction?
- When there’s no friction, how hard should we push so that we get the same acceleration as with friction?
Students made their predictions and we tested using the simulation. Now we can modify the equation we derived yesterday and change force to net force.
College-Prep Physics: Today we shared out the results from Friday’s PhET Force and Motion simulation lab. Some students used Desmos to determine the line/curve of best fit, other did it manually (easy since the data is perfect). Then we tried to tease out what each of the constants in the 2 equations mean. (Here’s the one time where it’s more useful to express the slope as a fraction rather than a decimal.)
College Prep-Physics: In the hallway, we attached the hooked ends of the tow-rope to handles on large demonstration cart. Then we attached a large demo spring scale to the middle of the rope.
With one student sitting on the cart, we pulled with a constant 30 N. The student riding on the cart dropped blue boxes of tape each second. We used an iPad metronome app to keep time.
“What do you notice about the pattern of blue boxes? What does that tell you about the motion of the cart?”
“What could we change about the set-up so that the spacing of the boxes is different?”
Next, we pulled with 50 N, and the student riding on the cart dropped red boxes of tape each second.
“What do you notice about the pattern of red boxes? What does that tell you about the motion of the cart? How is it like the blue boxes? How is it different?”
“What else could we change about the set-up so that the spacing of the boxes is different?”
Finally, we added a second student to the cart and pulled with the original 30 N. One of the students dropped yellow packs of post-it notes each second.
“What do you notice about the pattern of yellow post-its? What does that tell you about the motion of the cart? How is it like the blue boxes? How is it different?”
Then we drew motion diagrams, position-time graphs, and velocity-time graphs for all 3 scenarios. We are just getting a feel for how force and mass affect acceleration, and the large-scale demo is memorable. Tomorrow we will use Desmos and PhET sim to determine a more quantitative relationship.
NOTE: I was originally going to have the student drop sugar packets — Regular (white), Sweet-n-Low (pink), and Equal (blue) — instead of the boxes. Turns out our school cafeteria only has regular sugar. So I improvised with the boxes of tape refills.