Cambridge International A-Level Physics 9702: Frequent mistakes

Cambridge Principal Examiner Reports for Physics 9702 identify recurring errors in units, vectors, mechanics, waves and calculations. Unit conversions and magnitude checks are frequently overlooked.

Units and magnitude

SI prefixes and conversions

Candidates frequently make errors with SI prefixes, powers of ten, and unit conversions. Many fail to check if numerical answers have sensible magnitudes.

Fix: Learn prefixes (k, M, m, µ, n). 1 km = 10³ m. Check: does 0.001 N make sense for a force?

Vectors and trigonometry

Vector resolution

Persistent weakness: selecting wrong trigonometric functions when resolving vectors. Confusing sine and cosine (e.g. F_x = F cos θ, F_y = F sin θ for angle to horizontal).

Fix: For angle θ to horizontal: horizontal = F cos θ; vertical = F sin θ. Draw diagram. Identify angle.

Force as scalar

Treating forces as scalar quantities. Forces have magnitude and direction; resultant requires vector addition.

Fix: Forces are vectors. Resolve. Add components. Find resultant.

Mechanics

Projectile motion

Difficulty understanding how speed varies in 2D motion without air resistance. Misunderstanding velocity-time graphs; incorrect application of kinematic equations.

Fix: Horizontal component constant. Vertical: uniform acceleration. Speed = √(v_x² + v_y²).

Newton’s third law

Confusing Newton’s third law pairs. Action and reaction act on different bodies; equal in magnitude, opposite in direction.

Fix: Pair: A on B, B on A. Same type of force. Different bodies.

Neglecting forces

Neglecting all forces acting on a system. Missing weight, friction, tension, etc.

Fix: Draw free-body diagram. List all forces. Resolve.

Energy and work

Work vs. potential energy

Confusing work done with potential energy changes. Missing that when force is perpendicular to motion, no work is done.

Fix: W = F × d × cos θ. θ = 90° → W = 0. Work = energy transfer.

Waves

Stationary vs. interference

Confusing conditions for stationary waves with conditions for constructive and destructive interference. Different phenomena.

Fix: Stationary: same frequency, opposite direction, fixed boundaries. Interference: two waves meeting.

Elastic collisions

Signs of velocities

Care with signs of velocities. Relative speed of approach = relative speed of separation. Sign errors common.

Fix: Define direction. Use consistent sign convention. v_1 − v_2 before = v_2 − v_1 after (for 1D).

Calculations

Missing factors

Common mistakes: doubling percentage uncertainty in diameter (for radius); missing factor of 2 in kinematic equations (s = ut + ½at²).

Fix: Check formulae. Diameter → radius: divide by 2; uncertainty doubles. s = ut + ½at² not at².

Careless reading

Candidates often don’t read questions carefully enough—missing key information or misunderstanding what quantity is asked.

Fix: Underline “find”, “calculate”, “show”. Re-read.

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Based on Cambridge International A-level Physics 9702 Principal Examiner Reports (2014–2025).