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 using “by” and “on” subscripts, and find a force’s partner by swapping the names. (Guide: “Every force is half of an interaction”; Practice Q1)
- I can explain why the two forces of a pair never cancel, because they act on different objects. (Guide: Figure 1)
- I can explain why internal forces cannot change the motion of a system’s centre of mass. (Guide: “Internal forces and the centre of mass”; Practice Q7(b))
- I can state what “ideal string” and “ideal pulley” assume, and what each assumption lets me say about tension. (Guide: “Tension: a chain of third-law pairs”; Practice Q4)
Skills
- I can draw separate free-body diagrams for two interacting objects and mark the third-law pair between them. (Guide: Figure 1; Practice Q5)
- I can find the size and direction of the force an ideal string exerts on an ideal pulley. (Practice Q4)
- I can derive dT/dy = λg for a hanging string at rest from the forces on a small segment. (Guide: “Tension: a chain of third-law pairs”; Practice Q6(a))
- I can integrate dT/dy = λg, with the tension at the lower end as the starting value, to find tension anywhere in a hanging string, including one with varying λ. (Practice Q3, Q6)
Reasoning
- I can use the four-question test to reject false pairs such as weight and normal force. (Guide: “Testing whether two forces form a pair”; Practice Q5(d))
- I can compare the forces between two objects of very different mass, and explain why they are equal. (Practice Q2)
- I can use a fixed centre of mass to predict how far each object moves when only internal forces act. (Practice Q7(c), (d))
- I can judge when the ideal-string model is good enough and when it is not. (Guide: Figure 2; Practice Q6(e))
All ticked? Move on to Topic 2.4, Newton’s first law, or return to the course roadmap.