As we continue with our investigation of acceleration, I want to use today's warmup to carefully consider the use of our lab equipment, specifically the motion detectors. Though there is no computational aspect to this problem, there is a tremendous amount of mathematical reasoning and some critically important physical considerations to be made. By matching our thought work with our lab work, I hope to enhance students' ability to meet NGSS Performance Expectation HS-PS2-1.
The task is to match four scenarios (involving a block of wood, a ramp, and a motion detector arranged in different ways) with four possible velocity versus time graphs. I first reveal only the images of the scenarios and ask students what they think each shape represents. Students naturally see the triangle as a ramp and the double rectangle as the motion detector. Surprisingly, the square is the most elusive object - it takes a moment or so before they nominate, correctly, that this is an object that can slide up and down the ramp. Having established what's being portrayed, I move the discussion to considering the similarities and differences between the scenarios.
For the "uphill" scenarios, I add the clarifying information that the blocks are given just enough of a push to get them partway up the ramp - they will slide back down the ramp before reaching the the top. For the "downhill" scenarios, I get students to recognize that being released implies no push in any direction. After that, I distribute hard copies of the scenarios, with the graphs to be matched, to each student.
Students may work individually or collaboratively. I move around the room to check in with all students and to provide support and affirmation. After about five minutes or so, I give students a two-minute warning - we'll take a look at solutions in a short time.
While showing the solutions, I solicit student comments. In the solutions below, a student decides it would be best if he determines the position graphs first. While I'm sure it is not necessary to do so, I see how it helps this student make meaning of this problem. In order to clarify scenario d, he adds in an initial distance of '5' units to connect the scenario and the graph more clearly.
Before we return to collecting data for our acceleration investigation, I want to review the goals. To engage students in that process, I provide them with a free write prompt, giving them a few minutes to privately record their thoughts in their notebooks. After the free-write, I ask them to turn & talk to a neighbor before soliciting their thoughts on the board.
The red text is from one of my sections while the blue text are any unique ideas from the other class.
In a final delay before collecting data, I reveal an exercise to be done at the end of the lesson. In this 3-2-1 exercise, I want students to be reflective about their data collection process and to share good ideas for upcoming sessions of data collection. I briefly share the exercise on the board and encourage students to be mindful as they collect data.
By previewing this now, I increase the likelihood of a successful wrap-up at the end of class. At this point, I send students to the back of the room to collect materials and get set up for collecting data.
Students have previously received the acceleration lab handout and are aware of the goals. There are two kinds of stations - ramps with cars and interconnected masses - where student teams are trying to create accelerations of 1.00 m/sec2. Though there are only two kinds of stations, there are multiple set-ups, allowing for as many as six different lab teams to be supported.
Here we see two different teams setting up for their work:
One of those teams seems ready to roll:
This team has two interconnected masses, using gravity to cause the desired acceleration:
This student is using her phone to get a picture of her set-up to be used in her upcoming report.
Though this is at an odd angle, one can get a glimpse of a very linear velocity graph:
Students continue collecting data for most of the rest of this period. They are trying to hone in on a particular acceleration and also collect enough data to show the repeatability of their apparatus. With about ten minute left in class, I ask student to clean up and join me at the front of the room for our final exercise.
In the final few minutes, we return to the Smartboard to share insights and thoughts about the data collection process. I ask for students to volunteer and insights, observations, or lingering questions. Here is the set of responses I received from my two sections:
The last point is very important as many students assume the motion sensor is infallible and may misinterpret the resulting graph without critically thinking about the operation of the sensor. We end the day by focusing on this common issue. I ask those students who suggest this tip to map each of the segments of the velocity graph, shown above, to the actual experiment - both the motion detector and the moving object. The resulting comments are identified by the comments and arrows in the above image. This is a critical insight and the time we take to elaborate upon it prevents us from capturing any "nagging questions" (the final part of the 3-2-1 exercise). It's a good trade-off - this observation is perhaps the most important one students need to make about the operation of the motion detector.