Speed, Velocity and Acceleration | Distance and Displacement | IGCSE & O Level

Speed, velocity and acceleration explained for IGCSE and O Level Physics — distance and displacement, scalar and vector quantities, and why acceleration does not mean speeding up. Part 6 of the forces and motion series. O-Level Physics 6091 Paper 2 is on Thursday 29 October 2026, Paper 1 on Friday 6 November. You run one lap of a four hundred metre track in fifty seconds. Your average speed was eight metres per second. Your average velocity was zero. Not nearly zero. Exactly zero. And nothing about the running was a trick. There are two ways to take that. One is that a word has been redefined to make the answer come out strange. The other is that there is something about that lap you have never noticed — and that the second word exists because the first one throws information away. Distance tells you how far you went. Displacement tells you how far you got. Run a lap and those are 400 metres and nothing at all, and both numbers are used by somebody: your legs spent the 400, your position knows about the zero. Everything after that follows from the same split. Speed is distance over time and carries no direction. Velocity is displacement over time and carries one. And acceleration is the rate of change of velocity — which is the sentence that catches people, because velocity includes direction, so changing direction at a steady speed is accelerating. That is not a technicality. Four hundred kilometres above your head there is something holding one speed for years and accelerating the whole time, at nearly nine tenths of the gravity you feel standing on the ground. It is falling. It keeps missing. CHAPTERS 0:00 You ran 400 metres. Your average velocity was zero 1:27 Why anyone needs two words 3:00 Displacement, defined 3:28 Distance and displacement, drawn 4:34 Two numbers came apart 5:00 Scalar and vector 5:17 Scalar and vector, defined 6:51 What the arrows mean 7:47 Velocity, defined 8:30 Average speed and average velocity 8:54 Average velocity, defined 9:10 The lap and the straight road 9:52 The caveat 10:39 Acceleration is not speeding up 11:18 Three ways to accelerate 11:57 What is on the paper 12:13 The satellite 13:49 Does the heavy one go faster? 14:33 Five places this is already happening 16:48 What would change my mind 17:10 Something to take away 17:34 Next: two lines that mean opposite things WORK THROUGH IT YOURSELF Notes, the five definitions in the wording the exam wants, the lap worked through both ways, and a self-test with the marks broken out so you can see exactly where each one sits: https://claude.ai/artifact/NtZBas91Xp... Do the self-test again tomorrow rather than twice tonight. Half-forgetting it and coming back is what makes it stick. WHAT IS COVERED distance — displacement — scalar and vector quantities — speed — velocity — average velocity — acceleration — why a change of direction is an acceleration Every definition is given in the wording examiners mark, with the common trap named next to it. Displacement is not the distance travelled. Velocity is not just "how fast" — an answer without a direction is half an answer. Acceleration is the rate of change of velocity, not a change of speed. And two vectors of the same size pointing opposite ways do not add to double; they add to nothing. A NOTE ON DISPLACEMENT AND THE TWO SYLLABUSES Singapore 6091 uses displacement throughout its kinematics outcomes, including displacement-time graphs. Cambridge IGCSE 0625 does not use the word in this topic at all — there, velocity is defined as speed in a given direction. The card in the video is marked so you know which applies to your paper. Learn the idea either way: it is what velocity is built on. THE BONUS QUESTION A 400-tonne space station and a one-kilogram satellite orbit at the same height. Does the heavy one have to go faster? It is not on your syllabus and you will never be asked to calculate it — but you can already reason it out, and the answer is the same shape as the reason heavy things do not fall faster. Answer in the comments. THE FIVE WORDS — REVISION Every definition is shown again at the end, one every five seconds, then all five on one screen. Timestamps are in the pinned comment. ASK If anything did not land — any slide, any step, anything I went past too quickly — ask in the comments and say which slide. I answer every question, and there is no such thing as a question too basic. PREVIOUS EPISODE    • Newton's Third Law of Motion | Conservatio...   NEXT Speed-time graphs. Everything in this episode turns into a shape, and the area under the line turns out to be a distance. Subscribe and it reaches you when it lands. SUDDENLY OBVIOUS The whole series, in teaching order:    / @suddenlyobviousphysics   Physics for Cambridge IGCSE 0625, Cambridge O Level 5054, and Singapore GCE O Level 6091 (Singapore-Cambridge SEC K323 from 2027). #physics #olevelphysics #igcsephysics