Paper 1 Q1 — What actually restores the pendulum

Oscillations — the restoring force is the tangential component of weight, not tension

What the examiners said

“Some responses stated that (a component of) the tension gave the restoring force, but this is always perpendicular to the direction of motion… many… started with the assumption of SHM and tried to work backwards but generally were unable to do so.”

Try the question (reconstruction)

For a simple pendulum displaced by a small angle θ, which force provides the restoring force that drives the oscillation?

Examiner report: “Tension is always perpendicular to the motion, so it cannot be the restoring force.”

Explore — resolve the forces on the bob

Displace the pendulum. The weight is resolved into a radial part (balanced by tension) and a tangential part (mg sinθ) — the tangential part is the restoring force. Note it grows with θ.

Displacement angle θ (°): 20

SHM requires a restoring force proportional to displacement. mg sinθ ≈ mgθ for small angles delivers exactly that; tension never can.

Original reconstruction inspired by the misconceptions described in the Singapore-Cambridge GCE A-Level H3 Physics 9814 November 2023 Examiner Report. Not the actual examination question. For classroom exploration.

Made by lookang, using Claude Fable 5. More resources: iwant2study.org