FBD’s and Completing the Parallelogram

My lesson for doing a POGIL to learn about Free Body Diagrams and Completing the Parallelogram failed due to broken force probes and too few materials in general.  For the groups who had two working probes it started out well and I believe this could be a fantastic lesson if the materials were all there. 


The goal was to use spring scale force probes to measure the tension the held up a known mass.  With one force probe the weight can be inferred, then two force probes can show that the tension in each probe was reduced to about half.  From here, wedges of paper at different angles were passed out and the students would hold their probes in that orientation, measure the force, and then use the wedge on their paper to match the angle.  For all configurations, students use a scale (1 cm = 1 N) to carefully draw out scaled FBDs.  Students are guided to draw the assumed vertical force and connect dotted lines from the measured, angled forces to the top of the assumed vertical force (completing the parallelogram).


I would love to work on transforming this lesson into a 5 practices lesson to really use student responses to guide the model that we develop.  I think in the future this would be great as two lessons.  The first one to practice using the devices, measuring out the vectors and learning about how mass and weight are related by using different objects and making a graph.  The second would be this lesson where they look at forces from different angles. 


This lesson came from a long “head scratching” experience of how to help students discover free body diagrams, rather than just explaining the steps.  I really want to help students discover more and guide the work that we do and I’m very happy to read the 5 Practices to help with that.  I always thought Free Body Diagrams was a pretty dry topic to learn at first (though incredibly useful) and it took forever for me to come up with a lesson that puts the focus on the students doing science instead of just taking lecture notes.  I think a lot of discovery lessons via the 5 Practices, followed by Hewitt’s Conceptual Physics Worksheets can create a strong conceptual base that is student driven.  Future tasks are to develop strong student discovery based lessons to learn Motion Maps, Energy LOL Diagrams (Bar charts), and Momentum Bar Charts.


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