A Collision Lab
Lesson 7 of 9
Objective: Students will be able to estimate the speed of an object by applying momentum conservation to collisions.
Throughout this unit, students have been applying momentum conservation to a variety of example problems involving collisions. So, the goal for today's lesson is to put the law of conservation of momentum into action (HS-PS2-2). Specifically, students shoot foam darts at toy cars and then measure the resulting velocity (SP3-SP5 & SP8). I start the class with a ranking task before moving into the actual lab activity. Today's lesson ends with a fish bowl activity as their ticket out the door.
This lab requires the following materials for each group: dart gun, toy car, timing device, and meter stick.
This introduction activity is meant to gauge students' thinking and help them assess their individual level of understanding. I always have any introductory activity ready to go when students walk into my classroom to help with time management, so this ranking task is projected onto a screen at the front of the room before class even starts.
The ranking task shows the velocity of two cars before and after collisions. It then asks students to rank the different situations in terms of change in momentum. I choose not to give students a copy of this task and only project it because I am informally assessing students' prior knowledge. Students know that my expectation for this type of introduction is to write down the problem, their rankings and reasoning, along with their confidence levels, in their science notebooks.
Once the students are settled, I read the instructions to the activity. My reading of the instructions is to ensure students understand that class has started! I emphasize to students that they should work individually and take about five minutes to rank the momentum changes of the cars, explain their reasoning, and then assess their level of confidence. During these five minutes of work time, I walk around the room and informally assess how students are doing with simple glances at their work. Specifically, I'm looking to see if students recognize that the instructions ask students to rank the situations from most positive to most negative.
When the five minutes are over, I am back at the front of the room and ask students to raise their hand if they had a confidence level of 8 or above. This helps me gauge the percentage of my students that feel good about the material we'll be covering today. I make a mental note of those who are confident and use this as a guide to pace today's lesson. At this point, I reveal to the students the order: A=E, D, B=F=G, C, H.
Collision Lab Activity
After students have completed the ranking task, it's time to move into the lab activity. I know that my students will not finish the lab write-up in class today, so I allow them to choose partners they feel comfortable meeting outside of class. Groups of three seem to work best for this lab so that there can be a timer, a recorder, and a person in charge colliding the dart and toy car. After they have chosen their groups, each student needs to come to the front of the room to grab a collision lab and the necessary materials (see equipment section).
Students start by measuring the mass of the dart and toy car before observing the characteristics of an elastic collision. When the dart strikes the toy car, they measure the distance that the car moves and the time it takes the car to cover that distance. With this information, students are able to calculate the average velocity of the car over several trials. The students repeat the entire process with perfectly inelastic collisions. Finally, students calculate and analyze the momentum before and after each collision to see if momentum is conserved.
The procedure in the lab document is straight-forward, but I still make sure to circulate throughout the room and check-in with the groups. I'm offering feedback on their timing methods and ensuring they are making and recording the proper measurements with the toy cars. I also am their biggest cheerleader when it's time to fire the dart gun so it collides with toy car!
When there is approximately 10 minutes prior to the end of class (5 minutes left of the time I've allowed for this activity), I ask students to put everything back the way they found it and return to their seats. I also tell them at this point that the lab due date is one week from today.
Fish Bowl Closure
Students are back at their desks after cleaning their lab stations, so now I pass around small slips of paper (about the size of a standard sticky note).I use just a stack of blank colored paper and chop it up with a paper-cutter in the faculty lounge before class begins. I also use an actual fish bowl to make the name fit the activity, so that's empty and sitting on my desk at the start of class. And, to ensure all questions are appropriate and meaningful, I remind students of these requirements and read each slip of paper before going over them!
Each student should take at least one of these slips of paper so they can participate in our fishbowl closure activity. I explain to my students that they need to write down one question that they'd like answered about the general concept of momentum conservation or specifically about the lab. Each student must write something down, fold the slip of paper in half, and place it in the clear bowl at the front of the room.
I keep the room pretty quiet for this closure, as I want students to really reflect on what information they are feeling unsure about. I also don't want students to all have the same question, which is more likely to happen if students are able to share. In the past I've noticed that students are sometimes too shy to ask questions, so this activity should give them confidence by being anonymous. And, I pass around the slips of paper instead of simply handing a student just one so that if they choose, a student can take more than one slip for multiple questions.
This is a pretty fun activity and I read and answer the questions during the next class period. These are some of the questions I answered during that next class, and they show quite a variety!