Want an A? Stop Giving Marks Away: 16 Unit Traps | IGCSE & O Level Physics

You pick the right equation. You rearrange it correctly. You press the right buttons. And the answer is out by a factor of 1000, because one number went in wearing the wrong unit and nothing after it could recover. Here are 16 unit conversion traps that cost marks in IGCSE and O Level Physics, every one of them taken from a published examiners' report. The physics is not the hard part. Units, prefixes and powers of ten is the largest section in the corpus these come from: 101 separate complaints across 21 of the 23 exam sittings read, more than any topic section in the whole set. Nothing here is invented, and nothing here is a ranking — these are the things examiners chose to write about, which is not the same as the things students get wrong most often. WHAT IS COVERED prefixes (k, M, G, c, m) — m for milli against min for minutes — powers of ten in one step — hours, minutes and days to seconds — milliseconds off a timebase — reading a stopwatch display — centimetres, millimetres and kilometres to metres — grams to kilograms — mass to weight — celsius to kelvin — the temperature DIFFERENCE that must not be converted — answering in the unit the question asked for — the unit of your answer as a check — the answer with no unit on it at all — and a unit added where none belongs THE ONE THAT SURPRISES PEOPLE A temperature difference is the same number in celsius and in kelvin. A rise of 20 °C is a rise of 20 K. Adding 273 to it is the mistake, and it is the only place in this video where the correct move is to do nothing. THE ONE PEOPLE ARGUE ABOUT N m, N cm and N mm are all moments — a moment is force times distance, so any force unit times any length unit will do, and only the first is SI. N/m is not a moment at all. It is a spring constant. The slash names a different quantity, and no number in front of it makes that right. WORK THROUGH IT YOURSELF All 16, each one as a question you answer before the working opens, plus the five checks to run before you substitute and a one-page reference table: https://claude.ai/artifact/CM4DJgs5XU... FIVE CHECKS BEFORE YOU SUBSTITUTE Prefixes multiplied out. Time in seconds. Lengths in metres. Masses in kilograms. Temperature in kelvin. And one after you calculate: is the answer in the unit the question asked for? NEXT The rest of them. Rounding, rearranging, reading a graph, drawing a graph, answering the command word — one section at a time. SUDDENLY OBVIOUS Physics for Cambridge IGCSE 0625, Cambridge O Level 5054, and Singapore GCE O Level 6091 (Singapore-Cambridge SEC K323 from 2027).