During the previous lesson, students identified the relationship between work and energy. The goal for today's lesson is to apply that knowledge of work and energy (HS-PS3-3) in a lab activity. Specifically, students will use spring scales to determine the mass of different objects (SP5) and then explore the relationship between work done and the mass of an object (SP3, SP4, & SP8). In order to do this, they need to use the prior knowledge that work is equal to force times distance. This lab not only activates prior knowledge, but it gives students the opportunity to work independently by determining the mass added to each cart and the number of trials necessary to collect an appropriate amount of data. I start the class with a video before moving into the lab activity. Today's lesson ends with a whole-class debrief.
This lab requires the following materials for each lab group: 1 cart, 1 spring scale, 1 piece of string, several small masses, and a meter stick.
Because there are so many resources out there and my students will be active for the rest of the lesson, I decide to use this NASAeClips video to review work and energy. In particular, this video does a great job of giving some remediation on work and energy: concepts students will need throughout the lab activity. Before I start the video I make sure my expectations are clear. Students need to be sitting quietly, listening and watching the video, and taking notes on meaningful material. To me, meaningful material includes any reference to previously learned concepts, equations, vocabulary, and examples. I am telling my students these expectations as I'm on my way to start the video.
Since work and energy are fresh in the students' minds after the video introduction, students are ready to spring into today's limited-instruction lab. This is a 2 part lab, so I allow students to choose their own lab groups of 2-4 people. My students are mature and have a good rapport with each other, so I never have to worry about someone being left out of a grouping. After they have chosen their groups, they need to come to the front of the room to grab a lab and then go to an already organized lab station.
I announce to students that they should get started, as long as they have a cart, several masses, a piece of string, and a meter stick. They are already familiar with the expectation that they need to check their lab stations to ensure they have the right materials. It is my rule that if something is missing at the end of the class, that group is charged with the cost of the missing item. I find doing this holds kids accountable and ensures my materials don't fall into someone's pocket.
In today's lab, students are investigating the relationship between the work done on an object, its mass, and the distance it is raised. They start by designing a ramp and measuring the mass of the cart without any additional masses. Students then pull the cart to the top of the ramp vertically (not along the ramp) and use a spring scale to measure the applied force. The set-up of the ramp might confuse students since they are raising the cart vertically, but reminding students that this lab has a second part (during which the ramp becomes crucial) helps satiate students' curiosity. The students repeat this process after they vary the mass on the cart. This is all the data collection that students need, so they can now create a data table that includes the masses of the cart, applied forces, ramp height, calculated work, and the changes in potential energy. With this lab data, students should be able to draw and justify a conclusion between mass and work done.
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. This is a partial inquiry lab, so while there is a general procedure, how students organize their data and draw conclusions is up to the individual lab group. When there is approximately 15 minutes left in class (10 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 this is only the first part of the lab, so they need to hang on to the data for a few days before we move into the second part. The second part of the lab will delve into the concept of energy conservation, but I've chosen to stop students here for today because we haven't yet defined the meaning of conservation of mechanical energy.
The purpose of our debrief closure is to reflect on what students noticed during the lab activity. I usually guide the students through the discussion, but often they will jump in and ask each other questions. At this point in the year my students know they must show respect for others when they are talking with thoughtful listening.
I start our debrief by asking students what trends they noticed in the data collected. As students contribute their thoughts, I write them on the board. I choose to write so that the students can be engaged in their conversations. While I don't mandate that my students copy down the ideas that are shared, many of them will come up after class and take a photo with their phones. The photo then allows students to reflect a second time on our conversation when they go to work on their lab conclusions.
This time, our debrief led us to talk about possible sources of error. Some of my students measured only the distance along the ramp, not the height relative to the lab table or floor. This discussion opened up an opportunity for me to reference the video shown at the start of class and remind them that gravitational potential energy is based on height.