IB Physics: Work, Energy & Power FAQ

Start by identifying what's conserved — energy, momentum, or neither — then pick the matching equation. Use W = Fs cosθ for work, ½mv² for kinetic energy, and P = W/t or P = Fv for power. Sketch a quick energy-flow diagram before calculating — it catches sign errors examiners see constantly.

Work is the process of transferring energy from one store to another, measured in joules; energy is what's actually stored or possessed by a system at a given moment. Lift a book and you do work against gravity — the result is an increase in its gravitational potential energy store.

You need W = Fs cosθ for work, KE = ½mv² and GPE = mgh for energy stores, P = W/t = Fv for power, and efficiency = useful output ÷ input × 100%. These sit in the IB Physics data booklet — memorise how to rearrange them, not just recite them.

Power equals work done divided by time (P = W/t), or force times velocity (P = Fv) when the force acts along the direction of motion. Check units carefully — power in watts, force in newtons, velocity in m/s — mismatched units are the commonest mark loss on this topic.

Most marks vanish because students forget energy is only conserved in a closed system — friction, air resistance and sound all leak energy elsewhere. State where energy goes before writing an equation, and use KEi + GPEi = KEf + GPEf + Ewasted rather than assuming a clean, frictionless transfer.

Work done is the mechanism — a force acting through a displacement — while energy transferred is the outcome, measured in the same unit (joules) but describing the actual change in a system's energy store. In a frictionless system the two numbers match exactly; once friction's involved, work done includes energy lost as heat too.

The work-energy theorem states the net work done on an object equals its change in kinetic energy (W_net = ΔKE). Examiners use this on "how far before it stops" questions — set net work (often friction force × distance) equal to the initial kinetic energy and solve for distance.

Work, energy and power sit in Topic 2 (Mechanics) for both SL and HL, and the core formulas are identical. HL students later meet extra applications — rotational kinetic energy in Topic 6 and energy graphs in simple harmonic motion — that build on these same foundations but go beyond what SL is examined on.

Yes — work, energy and power regularly appear in Paper 3, usually inside a data-based question asking you to calculate efficiency, power output, or energy loss from graph or table data. According to the IB Physics guide, first exams for the current syllabus were 2025, and Paper 3 tests exactly this kind of applied, data-driven mechanics question.

Not conceptually hard, but it's a topic where careless sign errors and unit slips cost real marks. In my experience marking mocks, students understand the ideas fine — they lose marks converting units (cm to m, minutes to seconds) or forgetting cosθ in the work equation when force isn't parallel to motion.

Nail the definitions precisely, practise past-paper questions that combine energy conservation with kinematics or circular motion, and always show every step of working — IB mark schemes award method marks even when your final numerical answer is wrong. Command terms like "calculate", "state" and "explain" each expect a different depth of answer.

Work, energy and power questions rarely stand alone — expect them combined with projectile motion, circular motion (where centripetal force does no work), simple harmonic motion, or electrical power in circuits. Papers 2 and 3 often ask you to find velocity from kinematics first, then use that value to calculate kinetic energy or power.