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AP® Physics 1: Algebra-Based · Unit 2 · Topic 2.3

Newton's Third Law: Topic Checklist (Physics 1 2.3)

Specific “I can…” statements for Newton's third law in the algebra-based course, each linked to the guide section and practice question that tests it.

Work through the list without notes. If you cannot do a statement, follow its link, then come back. Guide = study guide; Practice = practice questions.

Understanding

  • I can state Newton’s third law as F_(A on B) = −F_(B on A) and list the five features of a force pair. (Guide: “Every force is half of an interaction”)
  • I can explain why the two forces in a pair never cancel each other. (Guide: “Why the pair never cancels”; Practice Q5)
  • I can say which force in this course acts between objects that do not touch, and that it also obeys the third law. (Guide: “Which forces act at a distance?”)
  • I can explain what makes a force internal or external, and how that depends on the system I choose. (Guide: “Internal and external forces”; Practice Q6)
  • I can describe tension as the pull between neighbouring pieces of a string, and define an ideal string and an ideal pulley. (Guide: “Tension: the string pulls both ways”)

Skills

  • I can name the partner of any force by swapping the two objects and keeping the type. (Guide: Figure 1; Practice Q2)
  • I can draw free-body diagrams for two interacting objects, with each pair split across the two diagrams and same-direction forces drawn side by side. (Guide: Figure 1; Practice Q5)
  • I can find the tension in an ideal rope pulled from both ends or tied to a wall. (Practice Q3, Q6)
  • I can use the centre of mass to predict where two objects that pull on each other will meet. (Guide: Worked example 1; Practice Q6)

Reasoning

  • I can compare the forces, and the changes in velocity, when objects of very different mass interact. (Guide: “Predicting with the third law”; Practice Q1, Q4)
  • I can explain how a tractor and trailer speed up even though the trailer pulls back on the rope as hard as the rope pulls it. (Guide: Worked example 2)
  • I can explain, in words, why tension varies along a rope or chain with mass but not along an ideal string. (Guide: “When tension is not the same everywhere”; Practice Q7)

All ticked? Move on to Topic 2.4, Newton’s First Law, or return to the course roadmap.

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