Work & Energy Lab: Part II
Lesson 5 of 10
Objective: After students do work on an object, they explore the conversion of potential energy to kinetic energy.
In the first part of the lab, students did work on a cart to increase its potential energy. The goal for today's lesson is then to examine how that potential energy is converted into kinetic energy (HS-PS3-3) in a lab activity. Specifically, students calculate the average velocity of the cart as it moves down the ramp (SP5) and then explore the relationship between work and kinetic energy (SP3, SP4, & SP8). I start the class with a ranking task before moving into the actual lab activity. Today's lesson ends with a hand signal closure.
This lab requires the following equipment for each lab group: 1 cart, 1 spring scale, 1 piece of string, a ramp, several small masses, and a meter stick.
When students walk into class today's ranking task is already projected onto the screen at the front of the room. I choose this specific task because it asks students to recall and apply the law of conservation of energy, which we covered in the previous lesson. Specifically, the task asks students to rank the final kinetic energies at the bottom of a ramp.
Once the students are settled, I read the instructions from the top of 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 5 minutes to rank the graphs, explain their reasoning, and then assess their level of confidence. During these 5 minutes of work time, I walk around the room and informally assess how students are doing with simple glances at their work. My changes in location help students stay quiet and focused.
When the 5 minutes are over, I reveal the answers to the students by writing them onto the front screen: F, A=B, C, D, E. I then ask if anyone got all of the answers in the correct order. This student has the correct solution and is willing to share with the rest of the class. He explains that his reasoning is potential energy and due to energy conservation the kinetic energy at the bottom of the ramp must be equal to the potential energy at the top of the ramp. Because potential energy is dependent upon mass and height, the student ends his explanation by showing a sample calculation using PE = mgh.
After he completes his explanation, I end the introductory activity by asking if anyone is in need of further clarification. Because the goal of this activity is to encourage students to bring mechanical energy conservation to their attention, the students keep their work to use as a reference throughout today's activity.
Work & Energy Lab Activity
Because this is a 2 part lab, I allowed students to choose their own lab groups of 2-4 people. They will return to the same groups that they formed on the first day of the lab, grab a copy of today's lab from the front of the room, 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, a ramp, 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 students accountable and ensures my materials don't fall into someone's pocket.
Students start by measuring and marking a distance of one meter from the bottom of their ramps. Then, they roll the cart down the ramp and record the time it takes for the cart to cover that one meter distance. In order to get accurate results, students need to complete several trials and calculate the average time. With this average time, students are able to calculate the cart's average velocity and kinetic energy. Students repeat this process for carts with different masses before discussing energy transformations, any relationship between mass and average velocity, and efficiency.
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 students write their conclusions, I expect the data to be neatly organized and clearly labeled. Also, the lab data that students collect must support any claims that students make in their discussions. For example, the average velocity varies significantly for each trial: the students should include potential theories about why this occurred. And, while the students writing supports their data, their lab should be much more explanatory about what is occurring in the lab and the science behind these events.
When there is approximately 15 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 when the lab is due, which includes the first part of the lab.
Hand Signals Closure
To assess how my students are feeling about applying conservation of mechanical energy, I ask students to show me hand signals. Before I share the prompt, I show students their 3 options: a thumbs-up for agreement, a thumbs-down for disagreement, and a flat hand for uncertainty.
Once the students are ready, I say "I can prove conservation of mechanical energy through the data collected in parts 1 & 2 of the work-energy lab." I then pause and repeat the prompt before asking students to share their hand signals, so that they have a moment to fully embrace and think about how they are feeling. The atmosphere is mostly quiet during this closure so that students are individually assessing their own level of understanding.
As students show their hand signals, I make a mental note of how many students are showing a flat hand or thumbs-down. Luckily, there was only 1 student who showed a thumbs-down, so I went over and spoke with him privately. Because he had been absent recently and was still not feeling well, he was confused and behind on the material covered. We found a time where he could come in for extra help and he was eventually able to apply the law of conservation of energy.