Conservation of Mechanical Energy

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Students will define and use the law of conservation of energy.

Big Idea

Students put their energy into learning about its law of conservation.


At this point in our energy unit, students have a solid understanding of work and energy fundamentals. In a previous lesson students identified the relationship between work and energy, and today's goal is to show the conservation of mechanical energy (HS-PS3-1). Specifically, students define kinetic and potential energies, review the work-energy theorem, and explore the energy transformations between these energies. The paired reading activity also includes defining conservative forces and a few example problems that apply mechanical energy conservation. 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 (SP8). Finally, students get to apply their new knowledge towards the end of class with collaborative problem solving (SP5).

This lesson is intentionally placed between a two-part work and energy lab. In part one, students explore the relationship between force, distance, and mass in terms of work and potential energy. Today's lesson introduces students to the conservation of mechanical energy and then allows them to practice its application in collaborative problem solving. Then, in the second part of the lab, students explore the relationship between force, distance, and mass in terms of work and kinetic energy. Also in this second part of the lab, students refer back to their energy calculations in part one and discuss the energy transformations and mechanical energy conservation.

Misconception Check

5 minutes

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 "The energy of a ball in free fall is created as it falls." The purpose of this statement is to get students thinking about mechanical energy conservation. Intentionally, there is no writing during this activity because I don't want to risk students copying down the misconception and then studying from it in the future.

I ask students to clear their desks and sit quietly for the activity because I really want them to think about what they know and how it applies to the statement on the board. 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 and that all answers must include a justification. 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 student 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. One students shares that "Energy must be created from the drop because it had an initial velocity of zero yet a non-zero final velocity." Then, I ask a student who did not have his hand up to share why he disagrees with the statement. The student states that "This was so eighth grade - energy is never created or destroyed." This process continues until a several students from both sides have shared their justifications. With this particular misconception, students focus on the semantics of the phrase, particularly the word "create," and discuss different interpretations of what it means to create.

Before moving into our final activity, I ensure that students understand that this is a BIG misconception. Energy cannot be created nor destroyed, and I justify that statement with an explanation of how a ball has gravitational potential energy converted into kinetic energy as it falls. 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.

Energy Conservation Paired Reading Activity

25 minutes

To learn how potential and kinetic energies are exchanged and mechanical energy is conserved, students engage in a paired reading activity. I introduce this activity by passing out the paired reading document to each student and explaining that I have already chosen their partners. Partners work best for this activity, and I have a list of who will be working together based on their current grade in the class. I don't tell the students how I've paired them, but I ranked the class by overall grade, split the list in half, and then matched the first names on each list. To save class time, I print this list and organize the pairs prior to class starting. Because there are an odd number of students, I make an exception and have one group of three. Pairing students forces them to work with someone different and ensures that ability levels are somewhat equal.

Students understand they don't need to scramble for a partner and have the document in front of them. I next 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. 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 exchange their annotations and copy down onto their document any ideas that they don't already have. For example, if one student thinks the definition of potential energy is important and her partner does not identify that, her partner needs to write the definition down on her own paper. The students then repeat this process until they have completed 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 3 or 4 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 has been redesigned to include more 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 long, 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 mechanical energy conservation.

After I'm done giving instructions, I reveal the pairs by simply reading them from the organized list. I have students move so they are sitting with each other, but since they are AP students I let them organize themselves and choose their own seat locations. Once everyone is settled I write 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.

Conservation of Energy Problem Solving

20 minutes

As closure and an informal assessment, students have the rest of the class to start tonight's homework. I call it informal because I don't want students to get nervous that it will count as a quiz or test grade. Since my goal is to assess how well they can apply mechanical energy conservation, I collect and grade the assignment for accuracy at the start of the next class meeting. Not only do I want to give students personalized feedback on this homework assignment, I also want to check the pacing of the course and make sure my students are ready to move on to the next lesson. Ideally, the homework assignment should look like this.

Students remain with their partners from the paired reading activity as I pass out a copy of the homework to each student. This is an assignment that needs to be completed by each individual, although they may use their partner as a resource while working in class. I encourage collaboration throughout their work time and since students are paired based on ability the "weaker" student is able to use the "stronger" student as a resource. It is also a goal of this strategy that the "stronger" student improves his or her knowledge by helping the "weaker" student work through the problems. Also, I walk around to offer help or problem solve with the students as they are working. You might hear me say "Well, what does your partner say about the kinetic energy at maximum height?" Or, I might say "Look back at the example on page 4, there's some very important information in there." My style is to lead the students to the answer, not just provide it for them. That being said, if a student is continually struggling and in obvious need of being shown the answer, I accommodate him or her.  

This is our closure activity for today and it's meant to have students apply their newly learned knowledge from the paired reading activity. Most students are able to complete the first 2 to 4 questions, depending on the individual. I am also trying to take a step towards a flipped classroom. I like students to have me as a resource when they work through problems, and I think it helps them build confidence. I find that a combination of work time (that lasts right up until the bell rings, since there wasn't a single student that was able to complete the assignment) and in-class learning best fits the needs of my students.