IGCSE Physics — Core: Measure It, Substitute It, Explain It
Start here
Lesson overview
Every Core chapter reduced to the quantities, the units and the one formula that carries the marks.
Go to lesson quiz →Exam guidance · Cambridge IGCSE Command-Word Lens
Pacing
~1 minute per mark; match answer depth strictly to `[1]`, `[2]`, `[3]`, or `[4]` mark allocations.
How IGCSE marks this
Command words govern mark schemes: 'State' = 1 concise fact; 'Describe' = what happens; 'Explain' = why/how using scientific terminology.
Watch out for
Vague everyday wording ('it gets bigger') fails Cambridge mark schemes where exact terms ('rate of collisions increases') are required.
Lesson content
General physics, forces and motion
Measurement comes first because it is free: a metre rule read at right angles to avoid parallax, a micrometer or vernier for a small length, the difference between systematic error (a zero offset, removed by correction) and random error (removed by repetition and averaging), and the technique of measuring many to find one — the thickness of a sheet from twenty sheets, the period of a pendulum from twenty swings. Scalars and vectors, mass against weight (W = mg with g ≈ 10 N/kg on the papers), and density as mass over volume with the units that trip candidates: 1 g/cm³ = 1000 kg/m³. Forces then supply the language: a force changes shape, speed or direction; the resultant of forces in a line; F = ma and the free-body diagram; momentum p = mv and its conservation in a closed system, with the safety arguments (increasing the time of impact reduces the force) that appear as 'explain, using physics' questions; the moment of a force as force times perpendicular distance from the pivot, and the principle of moments for a body in equilibrium; elasticity with the limit of proportionality, extension, and the fact that area under a force-extension graph is work done; and the pressure family: P = F/A for solids, P = ρgh for liquids — independent of the container's shape and of the surface area — with atmospheric pressure, hydraulics as pressure transmission, and upthrust explained by the pressure difference between the top and bottom of a submerged object. Energy is the chapter that ties the course together and it is examinable in one sentence repeated in every context: energy is conserved, never created or destroyed, and 'wasted' energy means dispersed to the surroundings where it cannot be used again. So the chain for any question is: identify the store that decreases, the store that increases, and where the dissipation goes; then use E = ½mv², E_p = mgh, E_k transfer and P = E/t, with efficiency as useful output over total input. Work done is force times distance in the direction of the force, and zero when the displacement is perpendicular — the reason no work is done carrying a bag horizontally.
Terms the papers use, defined as they must be written
SPEED vs VELOCITY: distance over time (scalar) against displacement over time in a stated direction (vector). NEWTON'S SECOND LAW: the resultant force equals mass times acceleration, F = ma. MOMENT: the turning effect of a force, force times the perpendicular distance from the pivot. LIMIT OF PROPORTIONALITY: beyond it, extension is no longer proportional to load. POWER: the rate of energy transfer, P = E/t, measured in watts. EFFICIENCY: useful output energy (or power) divided by total input energy (or power).
A falling-ball energy chain, written as the mark scheme wants it
A 0.20 kg ball is dropped from 45 m. Ignoring air resistance, find its speed on landing. Store change: gravitational decreases, kinetic increases, so mgh = ½mv² and mass cancels, giving v = √(2gh) = √(2 × 10 × 45) = √900 = 30 m/s. At Core you may substitute the numbers straight in — 0.20 × 10 × 45 = 90 J, then ½ × 0.20 × v² = 90 gives v² = 900. Now with air resistance the speed is less, because energy is dissipated to the surroundings as thermal energy of the air and the ball, and the paper asks for exactly that wording; 'energy is lost' earns nothing, since energy is not lost. If the ball reaches a steady speed, add: drag increases with speed until drag equals weight, the resultant force is zero, so acceleration is zero — terminal speed explained in four clauses.
Moments, upthrust and the two errors to avoid
A 2.0 m uniform plank of weight 60 N rests on a pivot at one end and is supported by a force at the other; a 40 N load sits 0.5 m from the pivot. Taking moments about the pivot: clockwise = 60 × 1.0 + 40 × 0.5 = 80 N·m; anticlockwise = F × 2.0; so F = 40 N. The error to avoid is using the plank's weight at the end instead of at its centre of mass — for a uniform plank that is the midpoint, which is why the 1.0 m appears. Upthrust, second part: a 500 cm³ object fully immersed in water displaces 500 cm³, of mass 0.500 kg, weighing 5.0 N — so the upthrust is 5.0 N and the apparent loss of weight is 5.0 N. If the object's own weight is 3.0 N it rises and floats with part above the surface, displacing only 300 cm³ when floating; the displaced volume changes with the situation, the object's volume does not.
Lesson summary
Core physics is measurement, units and about fifteen equations; the marks go to converting first, writing the formula, and stating the answer with its unit. Explain forces with the resultant and acceleration, motion graphs with gradient and area, and safety with force and time of impact or stopping distance. Energy is always conserved and never 'lost': name the store that decreases, the store that increases, and where the dissipation goes.
Mastery Check · Lesson Quiz · pass 60%
Pass this quiz to lock in spaced-repetition flashcards and unlock the next lesson.
Create a free account to take quizzes, earn checkpoints and track your scores — no card needed.
Create your free account