How to Master Physics Equations and Units

Cambridge IGCSE Physics 0625 expects you to recall most equations from memory: there is no full formula sheet. Around 40% of theory-paper marks involve calculation, and every calculation needs the right equation, the right units and the right rearrangement. Master those three skills and you protect marks across all six topics at once.

How many equations do you need to memorise for IGCSE Physics?

Core candidates need about 20 equations; Extended candidates need about 30. That sounds heavy, but the list clusters by topic: roughly ten in Motion, Forces and Energy, six in Electricity, and a handful each elsewhere. The full set, marked Core versus Extended, is in our 0625 equations list. Print it. It is the single highest-value page on this site.

Learn each equation in three forms together: words, symbols and units. For example: “speed equals distance divided by time”, $v = \dfrac{s}{t}$, metres per second. Cambridge mark schemes accept the symbol form, but the word form is what stops you confusing v-for-speed with V-for-voltage under pressure.

What is the fastest way to memorise them?

Daily low-volume retrieval beats weekly cramming. The drill takes five minutes:

1. Take a blank sheet. Write every equation for one topic from memory.
2. Check against the printed list. Mark gaps in red.
3. Tomorrow, start with yesterday’s red ones, then a new topic.

Cycle through all six topics in six days; rest or test on day seven. After three weeks, most students can write the full Core list in under ten minutes. Keep the habit until the exam, because recall decays in about two weeks without practice.

A warning about the triangle method. Cover-up triangles (V over I·R) work for three-quantity equations but collapse for anything bigger, like $E = mc\Delta\theta$ or $v = \dfrac{2\pi r}{T}$. Learn algebraic rearrangement instead: do the same operation to both sides until the unknown is alone. It is one skill that covers every equation, including the unfamiliar ones Extended papers like to spring.

Units: where the easiest marks die

Examiners report unit errors among the most common causes of lost marks in 0625. Three rules remove most of them.

Rule 1, convert before substituting. The equation list works in SI units: metres, kilograms, seconds, amperes. Questions deliberately give grams, minutes, kilometres or centimetres. Convert on a separate labelled line first. Key conversions: $1\ \text{km} = 1000\ \text{m}$, $1\ \text{g} = 0.001\ \text{kg}$, $1\ \text{minute} = 60\ \text{s}$, 1 kWh questions need power in kW and time in hours.

Rule 2, write the unit with every answer. A correct number without a unit usually drops the final A1 mark. Train the habit in homework, not the exam hall.

Rule 3, use units to sanity-check. Density in $\text{kg/m}^3$ should be 1000 for water. A car speed of $4000\ \text{m/s}$ means a conversion slipped. Speeds, masses and forces have realistic ranges, so learn a few anchors (walking $\approx 1.5\ \text{m/s}$, $g = 9.8\ \text{N/kg}$, mains voltage 240 V in Malaysia).

Do you really have to memorise, or can you derive?

Some students argue they can derive equations on the day. For a few, like average speed, that works. For most (refraction, e.m.f., specific latent heat) there is nothing to derive from; the definition is the equation. Memorisation is not the enemy of understanding here. Knowing $W = mg$ instantly frees your working memory to think about the actual problem. Students who pause to reconstruct equations run out of time on Paper 4, which gives roughly one minute per mark.

Track which equations cost you marks

Keep a one-page log. Each time a practice question goes wrong because of an equation or unit, note which one. Most students find three or four repeat offenders, commonly the pressure equations, $Q = It$ versus $E = VIt$ confusion, and forgetting to square the speed in kinetic energy. Drill those specifically. Want the audit done for you? Book a free 1-hour taught trial. We find your repeat offenders in the first lesson, then target only the equations leaking marks. 1-to-1, online, RM80/hr after the trial. WhatsApp us.

Pair this page with the calculation answer method: equation, substitute, rearrange, answer with unit. Equations get you to the start line; that method carries you to full marks. Spend week one of any revision plan on equations, because every later week depends on them.