Conceptual Understanding in Introductory Physics XXXV: Free-body Diagrams

Free-body diagrams are ubiquitous in introductory physics courses. They should be straightforward, but I’ve noticed that student frequently struggle with them at first because they want to include velocity or momentum in addition to forces.

For this question, choose an arbitrary (the more arbitrary, the better) physical situation. It could be something from your or the student’s immediate surroundings. Try not to use a situation from the textbook or from class discussion. It ideally needs to be something the student hasn’t seen before, but also within the context of the course.

  • Either the instructor or the student will pick one entity to be “the system.”
  • The student is asked to draw a correctly labeled free-body diagram for the chosen system, and to verbally describe each force using object-agent terminology. Each force should be represented by an appropriately drawn arrow and labeled in a notation that encodes its nature as “the force on ____ by _____” where the system goes in the first blank (the “object”) and the “agent” providing the force goes in the second blank. An example might be \mathbf{F}_{{\mathrm{Earth}},{\mathrm{Mars}}}, which would be verbalized as “the force on Earth by Mars.”
  • Is the net force on the system zero or nonzero? How do you know?
  • If the net force on the system is zero, identify the force that nulls out each individual force (I hope that wording makes sense…let me know if it doesn’t.).
  • Now change the situation by removing an entity that’s interacting with the system. How does the free-body diagram change? Be specific and as detailed as possible.

Feedback welcome!


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