An Atwood's Machine Inquiry Lab
Lesson 13 of 16
Objective: Students will explore the relationship between mass and acceleration of a pulley system.
In an earlier lesson, students solved problems that required them to apply Newton's Second Law, so our goal today is to apply that knowledge in a lab activity (SP3, SP5, & SP8). Specifically, students will determine the mathematical relationship between mass and acceleration of a pulley system (HS-PS2-1). I start the class with a 1-minute essay introduction, then we move into the lab activity before doing a hand signal closure.
This lab requires a the following materials: low-friction string, pulleys, various masses, meter sticks, and stopwatches. Prior to students arriving, I have a complete set of these materials at each lab station around the room.
To start class today, students come into the room and take out a blank sheet of notebook paper. On the board I've written "What is typically involved when solving two-body force problems?"
This is a 1 minute essay technique, where students get only 1 minute to answer the question and must work individually. The goal is to get students thinking about what they already know before they even start the lab on forces. I use a stopwatch, but because some of my students need a little time to process I usually give them about 90 seconds.
After their time is complete, I ask for volunteers to share their essays. I try to be a facilitator during this process and let students consider the responses of their peers. For example, if a student makes an assumption that is not valid I let another student speak up and question the invalid statement. I also encourage students to write down any valid ideas that they had forgotten to include.
The goal of this strategy was to remind students they need to find the acceleration of each object and the force acting between the objects. Once these concepts are shared in an essay, I emphasize their importance and move into the lab activity. Students keep their mini-essays as a reference for the lab.
Atwood's Machine Inquiry Lab
Students get to choose their partners for this lab since it will need to be completed outside of class time and I want to make sure they are comfortable contacting the people in their group. I suggest that students work in groups of 3, although I allow students to work in pairs or groups of 4 if they are more comfortable. One person from each group comes to get the materials and a copy of the lab. The lab sheet is purposely void of much information because students must create their own procedure and evidence their decisions.
Most lab groups start the lab by familiarizing themselves with the mass-pulley system, so this means they add different weights to the system and make sure the string is properly placed on the pulley. Then, students design a procedure that allows them to measure the acceleration of the system. Students have the freedom to choose how many trials they complete, which masses they use, and where to initially place the masses. The goal is that students see (through the use of kinematics and tensions) that acceleration due to gravity is consistent. After students design a procedure, they follow that procedure and record data. Most groups choose to record the time it takes for the masses to fall, mass distributions, and initial placements of each mass. After students are done collecting data, and if time permits, they start calculating the acceleration of each trial and writing a conclusion that explains their results.
Since the lab is open-ended, I make sure to circulate throughout the room and check-in with each of the groups. I offer students feedback on their calculations, reminding them of F=ma, and answer any questions. When there is approximately 10 minutes prior to the end of class (5 minutes left of the time I've allowed for this lab), I ask students to put everything back the way they found it and return to their seats. I also tell them at this point that the lab will be due one week from today.
This lab sample is a wonderful example of what I hope students are able to accomplish. This group clearly states their objective and did a nice job of outlining their procedures on page 1. Page 2 includes an organized data table, free-body diagrams, and calculations with assumptions. The conclusion, on page 3, is the only part of this lab that I would consider weak. I wish they had referenced the data and calculations to support the claims made in the first paragraph.
Hand Signals Closure
To assess how my students are feeling about applying Newton's Second Law in the Atwood Machine Inquiry Lab, I ask students to show me hand signals. Before I share the prompt, I show students their 3 options: a thumbs-up for agreement, a thumbs-down for disagreement, and a flat hand for uncertainty.
Once the students are ready, I say "I can apply and explain Newton's Second Law when given a 2-body situation such as an Atwood Machine." I then pause and repeat the prompt before asking students to share their hand signals, so that they have a moment to fully embrace and think about how they are feeling. The atmosphere is mostly quiet during this closure so that students are individually assessing their own level of understanding.
As students show their hand signals, I make a mental note of how many students are showing a flat hand or thumbs-down. Luckily, there was only 1 student who felt this way after the lab, so I went over and spoke with him privately. Because he had been absent recently and was still not feeling well, he was confused and behind on the material covered. We found a time where he could come in for extra help and he was eventually able to apply Newton's Second Law to 2-body systems.