F = ma 실수: Net 힘 vs 무게
Newton’s second law uses 순 힘 — the vector sum of all 힘s — not whichever 힘 you notice first.
F_net = m a — 순 힘 뉴턴 단위로; 질량 킬로그램 단위로; a m/s² 단위로
함정 1: Using 무게 alone when other 힘s act
틀림: Block on a table being pulled: set F_net = mg.
맞음: Horizontal pull problems: F_net is along the surface (pull − 마찰). 무게 and normal often cancel vertically.
수정: Sketch all 힘s, then sum components along the 가속도 direction.
함정 2: Forgetting 마찰 opposes the tendency to slide
틀림: F_net = F_pull when a rough surface clearly has kinetic 마찰.
맞음: While sliding, F_net = F_pull − μk N (same line, opposite directions).
수정: Static 마찰 can prevent motion entirely — then a = 0 and F_net = 0.
함정 3: Putting kilograms into F_net
틀림: m = 2 kg, a = 3 m/s² → writing F = 2 without 단위 thinking “2 kg of 힘.”
맞음: F_net = m a = 2 × 3 = 6 N.
수정: 힘 answers must be 뉴턴 단위로 when m is kg 단위로 and a m/s² 단위로.
함정 4: Confusing F = ma with W = mg
틀림: Any time you see 질량, automatically writing F = mg even when the question asks for horizontal 가속도 힘.
맞음: W = mg is 중력의 힘. F_net = ma is whatever 순 힘 생성합니다 the stated 가속도.
수정: Ask: “Is gravity the only 힘, or the 순 힘 I need?”
함정 5: Using a single 힘’s magnitude when 힘s cancel
틀림: Two equal opposite pulls of 10 N → claiming F_net = 10 N.
맞음: Equal opposite 힘s → F_net = 0 → a = 0 (if that is the full story).
수정: 순 힘 is a sum of vectors, not “the biggest arrow on the diagram.”
결론
F = ma needs 순 힘 뉴턴 단위로 and 질량 킬로그램 단위로 — 무게가 아님 alone, and not a single 힘 when others cancel or oppose it.
FAQ
무엇인가요 순 힘?
The vector sum of all 힘s on the 물체. That sum equals m a.
Is 무게 always the 순 힘?
Only in free fall with gravity as the only 힘 (idealized). Tables, strings, and 마찰 usually add other 힘s.
연습 W = mg · 힘 계산기 · 마찰 계산기 · 무게 계산기 · 경사면 계산기