IB Diploma Physics SL · Theme A.3 Work, energy and power
Mini-Lesson
Work, Energy & Power
This mini-lesson covers Theme A.3 — Work, energy and power: work done by a force, kinetic and potential energy, the conservation of energy, and power and efficiency.
Work through each screen, answer the questions as you go (some are reasoning, some are calculations) and collect ⭐ stars. Watch for the HL flag on higher-level extensions. Press Start when you're ready.
A.3 · work
Work done by a force
Work is energy transferred when a force moves its point of application. Only the force component along the displacement does work.
W = Fs cosθwork (J) = force (N) × displacement (m) × cos(angle between them)
If the force is perpendicular to the motion (θ = 90°, cos90° = 0) it does no work — e.g. the tension in a string on a mass in circular motion, or the normal force on a sliding box.
Quick check
Quick check
?A waiter carries a tray horizontally at constant height across a room. How much work does the upward force from their hand do on the tray?
Calculate
Calculate
#A person pushes a box with a steady 50 N in the direction of motion, moving it 8.0 m. Find the work done.
J
Hint: W = Fs cosθ, with θ = 0 so cosθ = 1 → 50 × 8.0.
A.3 · energy stores
Kinetic and potential energy
Two workhorse energy formulas:
Eₖ = ½mv² · ΔEₚ = mgΔhkinetic energy · change in gravitational potential energy
Elastic (spring) potential energy stores work done stretching a spring that obeys Hooke's law (F = kx):
Eₚ = ½kΔx²elastic potential energy in a spring of stiffness k
#A 1500 kg car travels at 20 m s⁻¹. Find its kinetic energy, in kilojoules (kJ).
kJ
Hint: Eₖ = ½mv² = ½ × 1500 × 20² = 300000 J; ÷1000 for kJ.
Calculate
Calculate
#Find the gain in gravitational potential energy when a 2.0 kg mass is lifted 1.5 m (g = 9.81 m s⁻²).
J
Hint: ΔEₚ = mgΔh = 2.0 × 9.81 × 1.5.
Sort it
Which unit measures it?
Tap an item, then tap the group it belongs to.
⚡ Joules (J)
🔌 Watts (W)
➡️ Newtons (N)
A.3 · power & efficiency
Power and efficiency
Power is the rate of energy transfer (or rate of doing work), in watts (1 W = 1 J s⁻¹). For a force pushing something at speed v: P = Fv.
P = W ÷ t = Fv · efficiency = useful output ÷ total inputefficiency is a ratio (× 100 for %) and is always < 1 for real machines
The wasted energy has not vanished — total energy is conserved. It is usually transferred to the surroundings as heat (thermal energy) and is no longer useful.
Calculate
Calculate
#A motor transfers 6000 J of energy in 4.0 s. Find its power output.
W
Hint: P = W ÷ t = 6000 ÷ 4.0.
Calculate
Calculate
#A machine takes in 800 J and delivers 240 J of useful energy. Find its efficiency as a percentage.