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Class 9–10 · Physics
Work, power and energy
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Work, power and energy
Lessons in this chapter
Lessons
1Work and direction
Work uses the component of displacement parallel to force. It is positive along force, negative against it and zero perpendicular to it. Holding an object still does no mechanical work on it despite biological energy use.
Key relationship
W = Fs cosθ; 1 J = 1 N m
Worked example
10 N over 4 m ⇒ 40 J; friction 3 N opposite ⇒ −12 J; net 28 J
Next lesson →2Kinetic energy
Motion gives an object kinetic energy. Doubling speed quadruples it. Net work equals the change in kinetic energy, obtained by combining F = ma with the constant-acceleration relation v²−u² = 2as.
Key relationship
Ek = ½mv²; Wnet = ΔEk
Worked example
m = 2 kg: at 3 m/s ⇒ 9 J; at 6 m/s ⇒ 36 J; required net work 27 J
Next lesson →3Potential energy
Potential energy belongs to a configuration. Near Earth's surface its gravitational change is mgΔh relative to a chosen level. A stretched spring also stores energy. Only energy differences matter in these comparisons.
Key relationship
Ep = mgh; spring energy = ½kx²
Worked example
2 kg lifted 1.5 m ⇒ ΔEp = 2 × 9.8 × 1.5 = 29.4 J
Next lesson →4Conservation of energy
Energy transfers and changes form. During an ideal fall, potential energy becomes kinetic energy. Friction transfers mechanical energy into thermal energy and sound; total energy remains conserved.
Key relationship
no resistance: mgh = ½mv² ⇒ v = √(2gh)
Worked example
h = 5 m ⇒ v = √98 ≈ 9.9 m/s
Next lesson →5Power
Power is the rate of doing work or transferring energy. The same task completed faster requires more average power. For aligned force and velocity, instantaneous mechanical power is Fv. Total energy also depends on operating time.
Key relationship
P = W/t; 1 W = 1 J/s
Worked example
600 J in 3 s ⇒ 200 W; in 6 s ⇒ 100 W
Next lesson →6Efficiency
Efficiency is useful output divided by total input. Practical devices transfer some energy into unwanted forms. Compare energy totals or powers over a consistent interval.
Key relationship
η = useful output/input × 100%
Worked example
Input 500 J, useful 350 J ⇒ η = 70%; other transfers = 150 J
Next lesson →7Energy resources
Sunlight, wind and flowing water are renewable resources; fossil fuel reserves are finite. Solar cells convert light directly, while hydropower drives a turbine. Evaluate environmental effects, reliability and storage rather than only the energy label.
Key relationship
E = Pt; 1 kWh = 3.6 × 10⁶ J
Worked example
100 W × 5 h = 0.1 kW × 5 h = 0.5 kWh
Next lesson →8Levers and pulleys
A lever balances moments about its pivot. A fixed pulley changes force direction; moving pulleys can distribute the load across supporting rope sections. A force advantage is exchanged for a greater effort distance, not free energy.
Key relationship
MA = load/effort; VR = effort distance/load distance; η = MA/VR × 100%
Worked example
Ideal moving pulley: 200 N load on 2 strands ⇒ effort 100 N; 1 m lift requires 2 m rope travel
Next lesson →Check your understanding
Try answering first, then open the question to compare your reasoning.
1. Triple speed: kinetic-energy factor?
Ek ∝ v² ⇒ 9
2. 20 N perpendicular to 5 m displacement: work?
W = 20 × 5 × cos90° = 0 J
3. 800 J in 4 s: power?
P = 800/4 = 200 W
4. 500 J input, 400 J useful: efficiency?
400/500 × 100 = 80%
5. Does mass affect ideal fall speed from a fixed height?
mgh = ½mv² ⇒ v = √(2gh), independent of m
Sources and scope
Original GetReady teaching notes. The 13 core chapters follow the existing site sequence. Physics to save life is an explicitly labelled older-edition supplement. Check your current book and board instructions for examination scope. Illustrations are schematic. Unless stated otherwise, use g = 9.8 m/s². A physics teacher should review content and edition coverage before it is relied on.