IGCSE Physics: Forces, Motion and Energy

Forces, motion and energy is the biggest topic in IGCSE Physics and underpins nearly every other area. You need to move fluently between kinematics graphs, Newton's laws, momentum and energy conservation — and explain them in words that match the mark scheme. This hub collects the key ideas, formulas and the mistakes examiners penalise most often.

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What you need to master

1Kinematics: distance, speed, velocity, acceleration and how to read distance-time and velocity-time graphs.
2Newton's three laws of motion, resultant force and free-body diagrams.
3Weight, mass, gravitational field strength and the distinction between them.
4Momentum, impulse, and conservation of momentum in collisions.
5Work, energy, power and the principle of conservation of energy.
6Hooke's law, elastic and plastic deformation, pressure and density.

Key formulas

v = u + at

s = ut + ½at²

F = ma

p = mv

W = Fd

KE = ½mv²

GPE = mgh

P = W/t

ρ = m/V

Examiner tips

•Always state the reference frame and direction for vectors — a bare number loses the mark.
•Quote Newton's laws in the exact examiner form: "the resultant force is equal to the rate of change of momentum".
•When using F = ma, check units before calculating (N, kg, m/s²).

Common mistakes to avoid

•Confusing weight (N) and mass (kg) — weight changes with gravity, mass does not.
•Forgetting that the area under a velocity-time graph is displacement, not velocity.
•Using v² = u² + 2as with a changing acceleration — the SUVAT equations only apply when a is constant.