Forces and Motion: F = ma Explained | IGCSE & O Level Physics

A steady push does not give a steady speed — Newton's second law, F = ma, and the reason time belongs in the answer. Force, mass and acceleration explained for IGCSE and O Level Physics. Push a box across the floor with a steady force and it slides at a steady speed. Hold the accelerator in one position and the car settles at one speed. Everything you have ever pushed says that force decides speed. Everything you have ever pushed was sitting on friction. When the box moves steadily, your push is not driving it forward. It is cancelling the friction. The two are equal, the resultant force is zero, and zero resultant force is the condition for constant velocity. You were watching friction balance out and calling it a law. Take the friction away and the same push never stops speeding the box up. So "how fast does a ten-newton push make something go?" is a question with no answer. You have to say for how long. Two lines you already have, put side by side: a = F / m, and v = a t, so v = F t / m Time. The speed you end up with is how hard you push multiplied by how long you push for. Force and time sit in that line symmetrically — half the force for twice as long arrives at exactly the same speed. Read it forwards and an ion thruster pushing with about ninety millinewtons, the weight of a couple of paperclips, outruns a chemical rocket, because it fires for years instead of minutes. Read it backwards and you get every safety device ever built: in a crash your mass and your change of speed are fixed, so force multiplied by time is fixed, and the only move left is to buy time. Airbags. Crumple zones. Bending your knees when you land. Catching an egg by moving your hands with it. Four things that look unrelated are one equation. Chapters: 0:00 A box you cannot move 2:06 Why you believe a steady push gives a steady speed 4:04 Take the friction away 6:12 The proof: v = (F/m) × t 8:40 The newton, defined 10:02 Read it forwards — the ion thruster 11:23 Read it backwards — airbags, crumple zones, knees 14:50 What would change my mind 15:34 Something to take away WORK THROUGH IT YOURSELF Notes, the four definitions in the wording the exam wants, a two-minute experiment with one coin, and a twenty-five mark self-test with the marks broken out so you can see exactly where each one sits: https://claude.ai/artifact/FWyQke4cNS... Do the self-test again tomorrow rather than twice tonight. Coming back to something after you have half-forgotten it is what makes it stick. WHAT IS COVERED Newton's second law — F = ma — resultant force — acceleration — the newton — friction — why speed depends on force and time Every definition is given in the wording examiners mark, with the common trap named next to it. F is the resultant force, not the force you happen to be applying. m is mass in kilograms, never a weight in newtons. Acceleration is the rate of change of velocity, not just a change. And an airbag does not absorb the force — it extends the time, which is not the same sentence and not the same mark. A NOTE ON THE ION THRUSTER The engine is NASA's NSTAR, which produces about 92 millinewtons at full power. It flew on Deep Space 1 and on Dawn, and Dawn ended up with the largest velocity change of any spacecraft from its own engines — around 11 km/s, built up over more than 2,000 days of thrusting. Launch rockets and gravity assists give more; no engine a spacecraft carries ever has. NEXT Momentum. The quantity on the right of that crash equation has a name, and this episode never used it on purpose. You have already derived the relationship. Episode 4 gives it a name and shows you what else it does. SUDDENLY OBVIOUS Physics for Cambridge IGCSE 0625, Cambridge O Level 5054, and Singapore GCE O Level 6091 (Singapore-Cambridge SEC K323 from 2027).