일 vs 전력 실수 (Joules vs Watts)
일 is 에너지 transferred (joules). 전력 is how fast that happens (watts). 시험 punish mixing them.
W = F d · P = W/t · P = F v — 1 W = 1 J/s · convert minutes → seconds
함정 1: Calling 전력 “일” because both use W
틀림: P = 50 W described as “50 joules of 일.”
맞음: 50 W means 50 joules every second. 일 would need a time: W = P t.
수정: 확인하세요 the unit: J → 일/에너지; W (watt) → 전력.
함정 2: Leaving time in minutes for P = W/t
틀림: 600 J in 2 minutes → P = 600/2 = 300 W.
맞음: t = 120 s → P = 600/120 = 5 W.
수정: Always convert minutes × 60 나누기 전에 for watts.
함정 3: Using P = Fv when 힘 is not along 속도
틀림: Any F and any v multiplied, even when 힘 is perpendicular to motion.
맞음: 사용하세요 the component of 힘 along the 속도: P = F v cosθ (or F_parallel × v).
수정: P = Fv is the special case when 힘 and 속도 are parallel.
함정 4: Mixing mechanical 전력 with electric P = VI
틀림: A lifting problem answered with P = VI because “전력 is VI.”
맞음: Mechanical: P = W/t or P = Fv. Electric circuits: P = VI (or I²R, V²/R).
수정: Match the 공식 to the context — motors can involve both, but the given data tells you which.
함정 5: Same 일, assuming same 전력
틀림: Two lifts of 100 J must both be “the same 전력.”
맞음: Same 일 in less time means more 전력: 100 J in 2 s → 50 W; in 10 s → 10 W.
수정: 전력 always needs a time (or a 속력 with 힘).
결론
Joules are 일; watts are joules per second. 변환하세요 time to seconds, and only use P = Fv when 힘 lines up with 속도.
FAQ
무엇인가요 the difference between 일 and 전력?
일 (J) is 에너지 transferred. 전력 (W) is the rate of that transfer: P = W/t.
How many watts in one horse전력?
소개 746 W for mechanical horse전력 in most school conversions.
연습 W = Fd · 연습 P = W/t · 일 계산기 · 전력 계산기 · Horse전력 계산기 · 전기 전력 계산기