Balancing at the Center of Mass

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Students will be able to define center of mass and analyze its impact in balance.

Big Idea

Students use a computer simulation to define the center of mass.


Now that students have a clear understanding of how torque works in a variety of situations from the previous lesson, today's goal is to explore center of mass. Students begin by watching a short video on the center of mass and then use a simulation to visualize how torque and center of mass are interconnected (SP2, SP3, & SP4). The simulation gives students (working in pairs) an opportunity to see how different variables affect the balancing of a seesaw. Finally, the lesson ends with students participating in an ABC summary that allows them to creatively express what they've learned.

Introduction with a Center of Mass Video

15 minutes

Because there are so many resources out there and my students will be active for the rest of the lesson, I decide to use this Khan Academy video to introduce the concept of center of mass. While we discussed the concept of center of mass on a very superficial level, this lesson requires students to define and reference the center of mass. It is the goal of this video introduction to provide students with a concise and straight forward look at how the center of mass is crucial to balancing an object. The video has detailed force diagrams and offers an example similar to what students experience in the next part of this lesson. Also, when I use a Khan Academy video I remind my students that this is a great resource to supplement the work we do in class.

Before I start the video I make sure my expectations are clear: students need to be sitting quietly, listening and watching the video, and taking notes on meaningful material. To me, meaningful material includes any reference to previously learned concepts, equations, vocabulary, and examples. While I do not collect their notes, these are an essential resource as students work through the simulation. I am telling my students these expectations as I'm on my way to start the video. 

Balancing Act Simulation Activity

30 minutes

After we finish watching the video, students use a computer simulation that allows them to be more active in their learning. The simulation allows students to predict the location of an object that balances a seesaw. Specifically, students start predicting (by using their new knowledge of center of mass) where a trash can must be placed to balance one or two fire extinguishers. Then, students use the simulation to determine if their predictions were correct. This first part of the simulation reviews forces in 1-dimension. Once they are done with that, students attempt to win a game that requires them to apply their knowledge of center mass under time constraints. After completing as many levels of the game as possible, students answer a few questions to debrief on the success and difficulties of playing that game.

Before we start the activity, I assign partners using the random student generator that already has my students' names loaded. By displaying the random generator on the front board, there is an element of suspense as the partners are assigned. Partners work best for this activity so that each student can be actively engaged throughout the work time. Because this activity does not need to be completed outside of class, I feel comfortable in choosing the pairings for my students.

We use MacBook Pro's in my district, so my students are familiar with how the computer and cart organization works. After each pair has been assigned, the students move to sit near each other, push their desks together, and grab a computer. As the computers are booting I pass out the balancing act activity so that each student receives a copy. This document directs them to PhET where they will be using the balancing act simulation. The activity sheet is also meant to direct the students in their learning so that they are confident in what material needs to be understood. However, the predictions, verifications, and answers to the questions need to be completed on a separate sheet of paper.

While students are working, I walk around to ensure they are actively engaged in the learning process. This means that they are on the proper website, reading or discussing some component of the simulation, and writing down appropriate information from the simulation. When I walk around, I'm spot checking their written work and engaging students in questions such as "Have you had time to explore the 'balance lab' tab with its mystery packages yet?" or "How many people and items can you cram onto the seesaw?" 

The students' written answers are the most important part of this activity, so I ensure they are thinking through how they will prove any claims they make. I do a lot of walking and questioning throughout their work time to ensure they can justify anything that has been written down. The AP Physics 1 exam places a lot of emphasis on justifying thought processes, so my goal in our dialogue is to practice with students how to successfully justify their arguments.

When there is approximately 10 minutes prior to the end of class (5 minutes left of the time I've allowed for this activity), I ask students to put the computers back on the cart and return to their seats. I also tell them at this point that the lab is due at the start of the next class meeting. Once everyone is back in their seats, we are ready to move into the closure activity for our lesson.

ABC Center of Mass Summary

5 minutes

Since we don't have much time today, as a quick closure I assign each student a letter of the alphabet. I usually choose the person closest to me and give her A, then the next person B, and so on. Students then have about a minute to think of a word that begins with their assigned letter to describe the introductory video or the simulation. After they've been given a minute to think, I point to the student who had A to share his word. Then, we go around the room until everyone has shared. In the past, if a student has struggled we pause and think of a word as a class to help that person out.  

This word list is an example of what students shared. Not only did it give me the satisfaction of knowing that they were able to reference topics from the video, but it also gave me insight into what students learned from the simulation. Some parts of the list that are in parentheses because it wasn't clear to me why the student chose that word and I asked for clarification.