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# Rotational Kinetic Energy & Momentum

Lesson 6 of 8

## Objective: Students will be able to define rotational kinetic energy and angular momentum.

## Big Idea: Students engage in paired reading to explore the concepts of rotational kinetic energy and momentum.

*40 minutes*

At this point in our rotational motion unit, students have a solid understanding of torque and center of mass. In the previous lesson students identified the relationship between torque and Newton's Second Law (HS-PS2-1) to define rotational inertia. Today, our goal is to apply that quantity of rotational inertia to rotational kinetic energy and momentum. I try to offer a variety of strategies to accomplish our goal, so the lesson starts with a misconception check and then goes into a paired reading activity (SP5 & SP8). Finally, students get to apply their new knowledge towards the end of class with a one word closure.

#### Resources

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When students walk into the classroom, I have a a misconception written on the board at the front of the room. Today's misconception reads "Constant net torque means constant angular velocity" and its purpose is to get students thinking about rotational motion in terms of torque and angular velocity.

I ask students to clear their desks and sit quietly for this activity because I really want them to think about what they know and how it applies to the statement on the board. Intentionally, there is no writing during this time because I don't want to risk students copying down the misconception and then studying from it in the future. When the students are ready, I read the statement and then ask students if they agree or disagree. I also remind them that in the world of AP Physics, an answer is not enough - we must justify everything. Then, I stay at the front of the room and just leave students to think for 2 or 3 minutes.

After time is up, I ask students to raise their hands if they agree with the statement written on the board. I take a mental note of these students before asking one of them to explain their reasoning. Then, I ask a student who did not have his hand up to share why he disagreed with the statement. This process continues until a several students from both sides have shared their justifications.

Before moving into our final activity, I ensure that students understand that this is a big misconception. Constant torque means there is no change in the angular *acceleration*, not that there is constant angular velocity. As we move into the next part of the class, I make a special effort to check-in with students that agreed with the misconception to see how they are feeling about things.

#### Resources

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To define the quantities of rotational kinetic energy and angular momentum, students engage in a paired reading activity. I introduce this activity by directing students to the paired reading document and explaining that I have already chosen their partners. Partners work best for this activity, and I already made a list of who works together based on their current grade in the class. I don't tell the students how I paired them, but I ranked the class by overall grade, split the list in half, and then matched the first names on each list. Because there is an odd number of students, I make an exception and have one group of three. Pairing students this way forces them to work with someone different and ensures that ability levels (of each group) are somewhat equal. To save class time, I printed this list and organized the pairs prior to class starting.

Now that students understand they don't need to scramble for a partner and have the directions to access the document, I share how this paired reading activity works. I expect that the students read one page at a time individually while annotating the text with information they deem important. Since they are reading the document on a Chromebook, students can save the document onto their desktop and take notes using functions in Adobe Reader. Students should stop reading when they get to the end of the page. Once both partners have come to the end of page 1, they should exchange their annotations and copy down onto their document any ideas that they didn't originally have. For example, if one student thinks the equation of linear kinetic energy is important and her partner didn't identify that, her partner needs to copy the equation into her notes. The students then repeat this process until they have gone through all seven pages of the document. They have approximately 25 minutes to work through this packet, so they should be reading and discussing each page every 6 or 7 minutes.

I share with students that this activity has three purposes. The first is to practice reading detailed information in a short amount of time. The AP Physics 1 exam includes a significant amount of reading, so I want students to be able to practice reading and pulling out important information under a time constraint. This parallels the second goal, which is to build students' stamina for reading physics material. At first glance the reading might seem a bit wordy, but it was intentionally selected to help students students prepare for the 3 hours they will spend taking the AP exam. The final goal is that students must grasp an understanding of the concepts, vocabulary, and equations used in defining rotational kinetic energy and angular momentum. If they complete this paired reading successfully, students should have a great study resource for rotational kinetic energy and angular momentum.

After I'm done giving the instructions, I reveal the pairs by simply reading them from my organized list. I have students move so they are sitting with each other; since they are AP students I let them organize themselves and choose their own seat locations. Once everyone is settled I put the end time of the activity on the front board and begin to circulate the room. My circulation lets me know if students are on task and allows me to redirect students if I hear misconceptions or off-task conversations.

As time for this activity expires, we move into the closure section of the lesson. Most students work efficiently to get through the reading, so as a class we are able to move into the group one-word closure after thirty minutes.

#### Resources

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To bring closure to our class today, students need to think of one word that they could use to describe their thoughts on rotational kinetic energy and angular momentum. Once a word is used by a student, it cannot be repeated. After I give them the instructions, students get about 30 seconds to think of their word. Then, we go around the room and share our words! I usually model the activity by being the first person to share a word, and then turn to the student closest to me to share next. We go up and down the rows until everyone has been able to contribute.

As students are sharing their words, I'm making mental notes on the overall tone of their comments. Are students excited and joking? Or, are the students monotone and overwhelmed? Listening to students as they share gives me insight into the paired reading and tells me if I need to make any adjustments. Also, if a word strikes me as being completely out of context, I do ask the student to clarify.

These sample words show me that the majority of students understand the fundamental concepts of angular momentum and rotational kinetic energy. The words came from the paired reading activity, so hearing students repeat them gives me confidence that they the material was effective. Reviewing these words, along with my informal observations and conversations that took place during the reading time, tells me that the activity is meaningful and I really don't need to make adjustments before repeating it next year.

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- LESSON 1: Talking About Torque
- LESSON 2: Torque on a Stick
- LESSON 3: Balancing at the Center of Mass
- LESSON 4: Center of Mass Lab
- LESSON 5: Rotational Inertia
- LESSON 6: Rotational Kinetic Energy & Momentum
- LESSON 7: Rotational Motion AP Practice
- LESSON 8: Rotational Motion Test