O-Level Pure Physics: Forces & Newton's Laws: Free-Body Diagrams That Score Marks
O-Level Pure Physics — Forces & Newton's Laws: Free-Body Diagrams That Score Marks: quick notes, common traps, and an exam-style example.
Newton’s 1st / 2nd / 3rd laws
1st: object stays in uniform motion / rest unless resultant force ≠ 0. 2nd: F = ma, where resultant force causes acceleration. 3rd: action–reaction pairs are equal in magnitude, opposite in direction, and act on different bodies. These three ideas drive almost every structured and free-response question in O-Level Pure Physics (Singapore syllabus).
Free-body diagrams
Always isolate one body and draw only forces acting on it: weight (mg, straight down), normal contact force (⊥ to surface), tension, friction (parallel to surface, opposes motion / impending motion). No arrows for velocity or acceleration — forces only. Once forces are labelled, resolve horizontally / vertically and apply ΣF = ma along each direction. For equilibrium, ΣF = 0 in both directions.
Common traps
Students lose marks by mixing up ‘reaction’ with ‘resultant’. Reaction ≠ Resultant. ‘Reaction’ in exam scripts often refers to normal contact force, not Newton’s 3rd law pair. Also, weight is always mg even on a slope — don’t write ‘gravity = 10N’, write ‘weight = 10N downwards’.
Exam habits
- Underline the object you’re analysing before drawing the diagram.
- Label forces with symbols (T, N, mg, f) and directions, not just arrows.
- State Newton’s 2nd Law explicitly: “ΣF = ma along x-direction”.
- For elevator / lift questions: apparent weight = N, not mg. Solve N − mg = ma.