Intro
This week is about making free-body diagrams and interaction pairs automatic. Before students solve harder dynamics problems, they need a durable habit of naming the system, deciding what counts as external, and drawing only forces that physically exist.
Core Lesson
A free-body diagram is not a sketch of motion. It is a map of forces acting on a chosen system. If the system is a box on a table, the forces belong to the box, not to the table, the floor, or your feelings about where the box is headed.
Interaction pairs matter because Newton’s 3rd law is waiting in the background. If the Earth pulls on the box, the box pulls on the Earth. If a rope pulls on the box, the box pulls on the rope. Those forces act on different objects, so they do not cancel just because they are equal in magnitude.
Many early mistakes come from inventing forces. “The force of motion,” “the force pushing forward because it is moving,” and similar ghosts do not belong on an AP diagram. Draw what is physically acting on the chosen system and justify each force in words.
AP Lift
The modern AP exam punishes fake forces and vague system language. Strong students can explain why two objects form an interaction pair, why equal-and-opposite forces act on different bodies, and why a diagram must represent forces on the chosen system only.
Must-Master Objectives
- Draw free-body diagrams for basic mechanics situations.
- Identify interaction pairs without saying they cancel on the same object.
- Explain why a force must have a physical source to belong on the diagram.
- Choose a clear system before discussing forces.
Problem Set Prompts
- Draw the free-body diagram for a book resting on a table.
- Draw the free-body diagram for a box pulled across a rough floor at constant speed.
- A student includes a “force in the direction of motion” on a sliding puck. Evaluate that choice.
- Identify the Newton’s 3rd law partner for the normal force on a box resting on a table.
- A rope pulls a cart to the right. What objects form the interaction pair behind that force?
- Why do the weight and normal force on a resting box not form a Newton’s 3rd law pair?
- Choose a system for a hanging mass on a string and justify it.
- Stretch: Draw separate free-body diagrams for each block in a two-block contact problem.
- Stretch: Describe one situation where friction acts opposite the direction you first expected.
Reflection Prompt
- Do you naturally start by naming the system, or do you still jump straight to arrows?
- Which free-body mistake feels most tempting right now: missing forces, extra forces, or bad interaction-pair logic?