Collisions

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Objective

Students will be able to explain how momentum is conserved in collisions.

Big Idea

The law of conservation of momentum gets applied to collisions.

Context & Equipment Needs

At this point in the Linear Momentum Unit, students have already defined momentum and impulse. During the previous lesson, we defined the law of conservation of momentum, so today's goal is to apply the law of conservation of momentum to collisions (HS-PS2-2). I start the lesson with an attention-grabbing video, before students take notes on collisions (SP5). The lesson closes with students applying their new knowledge in conservation of momentum collision activity (SP3, SP4, & SP8).

For each lab group, the collision activity requires 2 toy cars, a stopwatch, and a meter stick. In my classroom, students use their cell phones instead of a stopwatch.

Introduction with a Video

10 minutes

As soon as students come into the room and are seated, I play this video of a rather graphic car accident. I choose this video because all of my students drive, so it's my hope that the video is meaningful since it involves something they do each and every day. After the video has finished, I ask students to consider how momentum was conserved in this collision. The goal of my question is to have an informal discussion about momentum conservation so that students start thinking about how momentum is conserved in collisions.  

I allow the discussion to continue for about 5 minutes, as my students are eager to share stories about car accidents that they've seen or been in. I allow anyone to share, since I continuously strive for a fair and welcoming classroom environment. When there is a lull in the conversation, I ask students to think about how the mass and velocity can be conserved when the SUV is destroyed (as it was in the video). Without providing students the answer, I let that question go unanswered and use it as a way to pique interest before moving into the next activity.

Collision PowerPoint

25 minutes

At this point in the lesson, students take out a sheet of paper since it's time to take notes on how the law of conservation of energy can be applied in collisions. My students are operating under the expectation that they must write down key points from the presentation. This expectation of how to take notes has been outlined and ingrained in their learning since freshman year.    

I display the Collisions PowerPoint to help the students understand what they need to write down. As I'm showing the slides on the front board, I have a hard copy of the PowerPoint which includes teacher notes (viewable only when the file is downloaded). These notes help me to stay focused and ensure I mention key concepts as we progress through each slide.

The presentation starts with a definition of collisions and emphasizes the importance of momentum conservation. From there, students are introduced to the difference between inelastic and elastic collisions before working through four examples. The first example asks students to find the final velocity and lost kinetic energy for an inelastic collision. Because this is the first opportunity students have to analyze a collision, I identify the important information in the problem and then work through the solution on the board. For the next 3 examples, I give students about three minutes to analyze and solve the problems before working the solution on the front board.

While I describe this section as direct instruction, I usually have a lot of interaction with my students throughout the presentation and am constantly moving throughout the room to change my proximity. The students ask questions, participate in problem-solving, and connect to real-world examples to stay engaged the entire time.

Collision Activity

20 minutes

Students have watched a video on collisions and been introduced to the fundamental concepts of collisions, so it's time for students to create collisions of their own. For today's closure students need to pick a friend or two to work with, even though each student does his or her own work. After students have relocated to sit next to the people they choose to work with, someone from each group needs to get a copy of the lab from the front of the room.

The procedure in the lab document is purposely vague because students must think about how to best collect their own data. Students come to the front of the room and get their meter stick and toy car, since I've organized it prior to the start of class. I also tell students that at least one person in each group needs to use their cell phone stopwatch. Since we don't have much time and each toy car is roughly the same, I provide students with the mass of the toy cars by writing it on the front board.

The lab asks students to prove that the conservation of momentum applies to a head-on collision of toy cars. In order to do this successfully, students measure the velocity of each car before and after the collision. Most students do this by measuring the distance each car covered before the collision, and the time it took to cover that distance. Specifically, students place the toy cars next to the meter stick and put one student in charge of pushing the cars while another student is in charge of timing the cars with their cell phone timers. A third student is in charge of measuring the time post-collision and the student that originally pushed the cars now measures how far the cars rebound before coming to rest. After a few trials (most groups did 3 - 5 trials), students move on to calculate the initial and final total momentums.

As students are completing the lab, I move throughout the room to offer feedback on the data collection process, such as methods on how to calculate a car's velocity or what possible sources of error exist. Students understand the expectation that once they have completed collecting the data it's time to move into completing the lab write-up.

When there is about five minutes left before the end of the class hour, I ask students to take their final measurements and start to put their materials away. My students are pretty efficient at this point in the year, so five minutes is more than enough time to bring the activity to an end. I also tell students that they need to complete their calculations, describe their procedure, and discuss errors for homework tonight. I collect the completed labs at the start of the next class hour.