IB Physics: Conservation of Energy FAQs

The law states total energy in an isolated system stays constant — energy transforms between forms (kinetic, gravitational potential, elastic, thermal) but is never created or destroyed. In the IB Physics guide this underpins the work-energy theorem and appears explicitly in the Energy, Power and Climate Change topic.

Set total initial energy equal to total final energy, identify every energy store involved (KE, GPE, elastic, thermal), then solve for the unknown. Draw a simple before/after sketch first — students who skip this step almost always forget a term, usually the height reference point or a friction loss.

Energy conservation tracks total energy (which can change form but never vanishes); momentum conservation tracks total mass times velocity in a closed system with no external force. In collisions, momentum is always conserved but kinetic energy only is in perfectly elastic collisions — inelastic collisions lose KE to heat and sound.

Most lost marks come from forgetting a energy store (usually thermal/friction losses), mixing up height reference points in GPE calculations, or dropping units and significant figures under time pressure. Examiner reports repeatedly flag students writing $KE=PE$ without accounting for resistive forces stated in the question.

Write the conservation equation explicitly before substituting numbers — examiners award method marks for a correctly stated equation like $E_k+E_p=\text{constant}$ even if your final number is wrong. Never jump straight to a number; markschemes reward each labelled step separately.

The three repeat offenders: forgetting to convert cm to m (or g to kg) before substituting, ignoring stated resistive forces in the question stem, and confusing energy (joules) with power (watts) when a rate is implied. Examiner reports for Physics Paper 2 mention unit errors as a recurring cause of lost marks.

SL students apply conservation of energy to straightforward mechanics and thermal contexts; HL students must also apply it within circular motion, simple harmonic motion, and more demanding multi-step Paper 2 questions requiring calculus-free but multi-stage reasoning. The underlying law is identical — only the context complexity and mark allocation scale up.

It's frequently paired with circular motion (energy at the top/bottom of a vertical loop), simple harmonic motion (total energy in oscillation), projectile motion, and electrical circuits (power dissipated over time). Practising these combined questions on past papers is the single best way to spot your own recurring error pattern.

The concept itself is simple — energy in equals energy out — but exam performance is often weak because questions bury it inside multi-step problems with friction, height changes, or circuit contexts. Students who treat it as 'easy' often under-revise it and then fumble the accounting under exam pressure.

Work through past paper questions that combine energy with another topic (circular motion, SHM, electricity), not just standalone energy questions — that's where marks are actually lost. Redo any question you get wrong within 48 hours; that repetition is what fixes the habit, not just reading the markscheme once.

Core formulas: kinetic energy $E_k=\frac{1}{2}m{v}^2$, gravitational potential energy $E_p=mgh$, elastic potential energy $E_p=\frac{1}{2}k{x}^2$, and work done $W=Fd\cos \theta$. These are given in the IB Physics data booklet, so memorising them isn't required — knowing when to use each one is what's actually tested.

Repeated mark loss on one topic — like conservation of energy — usually means the concept is understood but the exam habit isn't fixed: forgetting a friction term, mixing units, or skipping the method-mark equation. Targeted practice on past paper questions, redone within days of the first attempt, closes this gap faster than re-reading notes.

The most useful thing you can do is encourage past-paper practice under timed conditions rather than passive note reading — physics marks come from applying formulas correctly under pressure, not from re-reading textbooks. Ask your child to explain their working out loud; if they can't explain a step, they haven't actually learned it yet.