Glancing Collisions

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Students will be able to apply momentum conservation to collisions occurring at an angle.

Big Idea

The vector nature of momentum helps students realize that the law of conservation of momentum applies to glancing collisions, too!


Throughout this unit students have become familiar with the concepts of momentum, impulse, collisions, and momentum conservation. So, today's goal is to expand students' understanding of momentum by applying the conservation law to glancing collisions (HS-PS2-2).

In the previous lesson students learned how to apply momentum conservation to head-on collisions, so class starts by asking students to review that material in an ABC Summary. I then apply that information by doing interactive examples before students collaboratively work through a problem set (SP5, SP6, & SP8).

ABC Introduction

10 minutes

To kick off class today, students are each assigned a letter of the alphabet and must come up with a word that starts with that letter and pertains to the concepts covered in the previous class. After about one minute of processing time, students share out their words with the rest of the class. I use this strategy as an introduction when I want to do a basic check for understanding and refresh of material, and today students did a nice job of summarizing what was learned with their words. When I hear students contributing words such as "deformation" and "toy truck" I see evidence of learning because those were words students used in the previous class. Some of their responses were most certainly creative, such as "ouch" and "Xenon," but I give them credit for at least having some fun with the activity.

Modeling Glancing Collisons

15 minutes

My students have already applied momentum conservation to collisions, so I don't give a traditional set of PowerPoint notes today. Instead, I explain how momentum conservation is applied to 2-dimensional collisions by working through 2 examples. Students are still expected to write down the problems and solutions in their science notebooks, as they do when we take traditional notes.

I start by projecting the first example problem onto the front board by placing it under the document camera. I've intentionally left blank space under the question so that I can work the problem out with the students and they can listen to my thought process. The students are certainly not passive throughout this activity, as I constantly ask them to contribute information and equations or perform calculations. In solving the first problem, I might ask students to explain why the horizontal momentum of the car is equal to zero or which trig function we'd need to find the direction of the resulting velocity.

For the second example problem, I give students a 5 minute head-start before I go through the solution. I want students to have an opportunity to self assess and attempt the problem without any assistance. Until I provide the solution to this second example problem, students are discussing possible solutions and calculating collaboratively with those seated around them.

The goal of this activity is that students get 2 examples of how momentum conservation is applied to glancing collisions. These completed problems then serve as a reference when students move into the collaborative problem solving portion of the lesson.

Glancing Collisions: Collaborative Problem Solving

25 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 the application of momentum conservation to glancing collisions, 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, and today they have freedom to work with anyone of their choice. 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 discussed in the example explanation portion of the lesson, and I want to give students an opportunity to practice with peer and teacher support.

I've chosen 6 problems from the text. The first 2 are straight-forward applications of momentum conservation and are almost identical to the examples. 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 having an object that with an initial, angled velocity). The final 2 problems require much more thought and are meant to challenge students with applications that include impulse and energy concepts. I've purposely assigned the problems in this order to build the students' confidence when solving 2-dimensional collision problems.

All of my students are able to finish the assignment before the end of class, and some groups even have time to double check their work. When the bell rings, students hand me their work on the way out the door and they leave the class with a rare, homework-free afternoon. I (jokingly) tell them as they are leaving that there isn't any homework tonight so they can rest-up for tomorrow's collision lab.