Work and Power

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이 개념을 다룬 실제 CSCA 기출문제입니다. 답을 펼치기 전에 먼저 직접 풀어 보세요.

1. CSCA Physics December 2025 Past Paper
   A horizontal pulling force F of 20 N is applied to an object of mass 10 kg . The object moves 5 m on a rough horizontal surface with a friction force $f = 5 \mathrm {~N}$. The work done by the pulling force $F$ is ( ).
   • A．100 J
   • B．75 J
   • C．- 25 J
   • D．25 J
   정답 및 풀이 보기

   정답: A

   1 Work by the pulling force: $W = F s \cos 0 = 20 \mathrm {~N} \times 5 \mathrm {~m} = 100 \mathrm {~J}$.
   CSCA Physics December 2025 Past Paper 전체에서 보기
2. CSCA Physics March 2026 Past Paper
   A horizontal force F pulls an object 5 m , doing 50 J of work. F equals
   • A．5 N
   • B．10 N
   • C．15 N
   • D．20 N
   정답 및 풀이 보기

   정답: B

   $W = F s$ (same direction, $\left. \cos 0 ^ { \circ } = 1 \right)$. $F = 50 / 5 = 10 \mathrm {~N}$. Answer: $B$.
   CSCA Physics March 2026 Past Paper 전체 풀기 — 시간 측정 및 채점
3. CSCA Physics April 2026 Past Paper
   As shown in the figure, a tower crane uses a steel cable to lift a load of mass $m$ vertically with acceleration $a$ through a height $h$. The gravitational acceleration is $g$. Which statement about this process is correct?
   • A．Work done by the cable tension is $m ( a + g ) h$
   • B．Work done by gravity on the load is - mah
   • C．Work done by the net force on the load is $m g h$
   • D．The load's gravitational potential energy decreases by $m g h$
   정답 및 풀이 보기

   정답: A

   Newton's second law in the vertical direction: $T - m g = m a \Rightarrow T = m ( g + a )$. (A) $W _ { T } = T \cdot h = m ( g + a ) h$ - correct $\checkmark$. (B) Gravity does negative work: $W _ { G } = - m g h$, not - mah - wrong. (C) Net-force work: $W _ { \text {net } } = ( T - m g ) h = m a h$, not $m g h$ - wrong. (D) The load rises by $h$, so gravitational PE increases by $m g h$ (not decreases) - wrong.
   CSCA Physics April 2026 Past Paper 전체 풀기 — 시간 측정 및 채점