A7LAM CSCA ACADEMYCSCA Academy / Physics / Mechanics
CSCA Physics · Lesson 3 of 16Work, energy and power
Travail, énergie et puissance · الشغل والطاقة والقدرة
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Energy is conserved: it changes form but the total stays the same. This idea solves problems that forces alone make hard.
Key ideas
- Work W = F·d·cos θ, in joules (J).
- Kinetic energy KE = ½mv². Gravitational potential energy PE = mgh.
- Without friction, KE + PE stays constant.
- Power is the rate of doing work: P = W/t; for a moving object P = Fv.
Formulas to know
- KE = ½mv²
- PE = mgh
- Power P = W / t (watts)
Worked example
A 2 kg ball is dropped from 5 m (g = 10 m/s², no air resistance). Find its speed just before it lands.
- Energy at the top: PE = mgh = 2 · 10 · 5 = 100 J.
- At the bottom all of it is kinetic: ½ · 2 · v² = 100.
- v² = 100.
Answer: v = 10 m/s
Practice questions
1. Work done pushing a box with 50 N over 4 m (force along the motion)?
12.5 J · 54 J · 200 J ✓ · 400 J
W = F·d = 50 × 4 = 200 J.
2. A 2 kg ball moves at 3 m/s. Its kinetic energy?
3 J · 6 J · 9 J ✓ · 18 J
½ × 2 × 3² = 9 J.
3. A machine does 600 J of work in 3 s. Power?
200 W ✓ · 1,800 W · 600 W · 0.005 W
P = W/t = 600/3 = 200 W.