Intro
This week turns free-body-diagram habits into Newton’s-laws reasoning. The goal is to use system choice, net force, and interaction language correctly enough that equations become explanations instead of detached algebra.
Core Lesson
Newton’s 1st law explains what happens when the net force is zero: velocity stays constant. Newton’s 2nd law connects the net external force to acceleration. Newton’s 3rd law tells you interactions come in equal-and-opposite pairs acting on different objects.
These laws only become useful when the system is chosen carefully. If you mix forces from different objects or treat a 3rd-law pair as if it acted on one body, the algebra can look clean while the physics is wrong.
The clean workflow is: choose the system, draw the free-body diagram, identify the net force direction, and then connect that to acceleration. The laws are reasoning tools, not slogans to recite after the math is done.
AP Lift
The modern AP exam cares about Newton’s-law language. Students should be able to defend why net force is zero or nonzero, why acceleration points where it does, and how Newton’s 3rd law differs from “balanced forces.”
Must-Master Objectives
- Distinguish Newton’s 1st, 2nd, and 3rd laws conceptually.
- Connect net force direction to acceleration direction.
- Explain why Newton’s 3rd law pairs do not cancel on one object.
- Use system choice to justify a dynamics setup.
Problem Set Prompts
- A puck slides across nearly frictionless ice. Which of Newton’s laws best explains its motion?
- A cart speeds up to the right. What must be true about the net external force?
- A car moves right while slowing down. What must be true about the acceleration direction?
- A student says equal-and-opposite forces always cancel. Evaluate the claim using Newton’s 3rd law.
- Why do the table’s upward force on a book and Earth’s downward force on the book not form a 3rd-law pair?
- If the net force on an object is zero, what are the possible states of motion?
- Compare a system defined as “just the block” with a system defined as “block plus rope.” What changes?
- Stretch: Explain why two colliding skaters exert equal forces even if one skater accelerates more.
- Stretch: Describe a scenario with nonzero velocity but zero net force.
Reflection Prompt
- Which Newton’s law feels easiest to say but hardest to apply correctly?
- When a dynamics problem gets messy, do you trust the system choice enough to slow down and rebuild from net force?