Law of Conservation of Momentum

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Students will be able to define and apply the law of conservation of momentum.

Big Idea

Students expand their knowledge of momentum when they apply the conservation law in a pick-a-problem quiz.


At this point in the Linear Momentum Unit, students have already defined momentum and impulse, so today's goal is to introduce the law of conservation of momentum (HS-PS2-2). I start the lesson with a group warm-up activity that reviews Newton's Third Law, before students engage in a reading exploration activity (SP8). The lesson closes with students applying their new knowledge in a pick-a-problem quiz (SP5).

Newton's Third Law Refresh Activity

15 minutes

Since students need to apply Newton's Third Law in today's lesson, I have a little competition to start the class and get students engaged in the lesson. As students enter the room, I have the "Check Your Understanding Questionsfrom the Physics Classroom loaded and ready to be displayed. Since my students sit at lab tables, I have pre-made groups of 3 or 4 students and ask each group to take out a blank sheet of paper. Students are sitting close enough at their lab tables that I don't need them to move, but I do give them a warning that they are to talk softly throughout today's warm-up. My emphasis on not talking loudly ensures that students are only working with their pre-made groups since this warm-up activity is a competition between those different groups.

Once the groups are settled and students have their paper ready, along with a pen or pencil and a calculator, I share with students the goal of our warm-up: to review and apply Newton's Third Law. I also tell students that I'll be showing them 6 different problems, and each problem will only be visible for one minute each. Their task is to read the problem, discuss the solution, and then write down their solution on that blank sheet of paper. Because this is a method of activating prior knowledge, I have no worries about the time constraint for each problem.

As I show each problem, I stay at the front of the room and remain quiet. I want the groups to work on their own, and I keep strict time with a stopwatch. My transition from one problem to the next is simply me saying "next" and then moving on to reveal the problem that follows.

After students have seen all 6 problems, I then go back to the first problem and reveal the answer (available by clicking on the "see answer" button). I give students a moment to check their answers with the given answer and ask any questions before moving on to reveal the next answer. The students are informally grading themselves as we go through the solutions while also refreshing their memories of Newton's Third Law. After each answer is revealed, I allow each group to keep their answers so they can refer back to any concepts needed as we move into the next portion of the lesson.

Reading Exploration Activity

15 minutes

Now that students have reviewed Newton's Third Law during our warm-up activity, they are ready to read about momentum conservation. At this point in the year I've developed trust in my students, so I allow them to choose a partner and take a computer from the cart at the front of the room. We use MacBook Pro's in my district and each department has a cart that can be shared, so my students are familiar at this point with how the computer and cart organization works. I decide to let students choose their partner since I want them to be comfortable and focus on internalizing today's material.

As the computers are booting and after students have pushed their desks together with their partner, I give each student a copy of the reading guide. This document is a guideline that helps students identify the absolute minimum material they should record into their physics notebooks. Once they've opened the document, the students follow the link to the Momentum Conservation section of the Physics Classroom. At this website and still in their pairs, students read through the text and use the document to guide them through the reading. I let students know that they can read aloud or individually, but the point of them being in pairs is to discuss and determine the most important parts of the text.  

While students are working, I walk around with the answer key to ensure they are actively engaged in the learning process. To me, this means that they are on the proper website, reading or discussing some component of work, and writing down a thorough set of notes. When I walk around, I spot check their written work and engage students in questions such as "So how do you define conservation of momentum?" or "What is a real-word example of conservation of momentum?" I also use this time to build rapport with students and attempt to make the material meaningful to the individual. For example, if I know a student is currently in lacrosse season, I might ask him or her "How is momentum conservation involved when the ball comes in contact with the stick?"

After about fifteen minutes, students are able to move on to the collaborative problem solving portion of the lesson. I'm not strict about ending the reading exploration activity at exactly fifteen minutes, so students can continue to work for a few more minutes if they need extra time.

Collaborative Problem Solving

15 minutes

Students are given a significant amount of time to work on today's informal assignment from our textbook. I call it informal because I don't want students to get nervous that the entire assignment counts as a quiz or test grade. Since my goal is to assess the students' level of understanding on momentum conservation, I collect and grade the assignment for accuracy at the end of class. 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.

Students choose to work with one other student, although they usually work with the same partner from the reading exploration activity. This is an assignment that needs to be completed by each individual, although they may use their partner as a resource. I encourage collaboration throughout their work time and I walk around to offer help or problem solve with the students as they are working. These problems represent the concepts covered in today's reading, and I want to give students an opportunity to practice application with peer and teacher support.

I've chosen 6 problems from the text. The first 2 are straight-forward applications of momentum conservation. The third and fourth problems are still cut and dry, however they have multiple parts and require students to think about slight differences in their calculations (such as a change in direction or change in mass). The final 2 problems require much more thought and are meant to challenge students. I've purposely assigned the problems in this order to build the students' confidence when solving momentum conservation problems and in anticipation of our pick-a-problem quiz closure.

I am also trying to take a step towards a flipped classroom.  I like students to have me and their peers as a resource when they work through problems, and I think it helps them build confidence. In the past I've attempted to do entire class periods of a full flipped classroom, but it's hard to hold the students accountable for digesting the needed material.  I find that a combination of work time (that lasts right up until the bell rings) and in-class learning best fits the needs of my students.

Pick-A-Problem Quiz Closure

5 minutes

Today's closure forces students to evaluate their level of confidence on applying the law of conservation of momentum. With about five minutes left in class, I notify students that they must select one of their completed problems to be graded. They make this selection by circling the problem, and that problem is specifically graded as if it were on a quiz. To me and my students, this means that they must include the equation, substitution of values, and final answer with units. 

I've avoided telling students this information until the last moments of class. In the past, students have spent far too long concentrating on only one problem and not finishing the assignment if they knew ahead of time about the pick a problem quiz. I want students to work through the entire assignment because each problem is a valuable opportunity to review and apply concepts. Also, I grade the entire assignment to offer individualized feedback, so it's crucial that students are motivated to do the entire assignment.

After students are aware that they must choose 1 problem to count towards a quiz grade, I give them a few minutes to choose the problem and make any necessary adjustments. They still have the ability to collaborate throughout this time, so I encourage students to check each other's work. However, time is up when the bell rings and students must hand in their completed problem sets as they leave the room.