In a typical physics class, the first concept students find confusing is the the superiority of position to distance for motion analysis. This Desmos activity (teacher.desmos.com/activitybuilder/custom/5c2a696c22098643949e9b74) may help, especially if you teaching remotely this fall.
The first video in the activity (seen above) shows two people walking toward each other. but cuts short. The goal is to gather and analyze data to predict the time and place that they will meet. My students have already used this platform to analyze the motion of a single walker, so they already know how to use this platform. You can easily modify this activity if your students are using Desmos for the first time. Using the Dashboard feature, you can watch students in real time as they graph the motion of the first walker. Then, as the students start to graph the motion of the second, you will notice most graphing with distance traveled. At this point, I project the graphs being produced on the front board.
I then lead a class discussion on how to get the graphs to predict time and place of collision. First we notice that the lines need to cross and so one has to have a positive slope and the other negative. That means that the second walker has to start at a "higher number". So naturally, that number should be the yard marker on the field. In other words, students discover "position" is better than "distance" in a meaningful way. Once students correct their data, most make a successful prediction.
I have found this activity useful in helping students understand why we use position and not distance in our future analyses.
Other Desmos activities at https://teacher.desmos.com/collection/5cba1ada3d8baa0c055fd5ba
