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A2 Daily A Level Physics question

2026-07-01 OCR A Simple harmonic motion (SHM): energy interchange OCR-A 5.1.1 Simple harmonic motion: energy transfer between kinetic and potential energy; dependence on displacement OCR-A 5.1.1 Simple harmonic motion: phase and displacement–velocity–acceleration relations (qualitative)

A low-friction glider on an air track is attached to a spring and oscillates with constant amplitude A in simple harmonic motion. At an instant the glider is at displacement x = A/2 from equilibrium. Which statement must be true at this instant?

  1. A The kinetic energy equals the potential energy at that point.
  2. B The speed is half the maximum speed achieved during the motion.
  3. C The kinetic energy is three times the potential energy at that point. (correct)
  4. D The restoring force on the glider is zero at that point.

Answer

The correct answer is C.

Correct: C — The kinetic energy is three times the potential energy at that point. At x = A/2, the potential energy is proportional to x^2, so it is 1/4 of the total; hence the kinetic energy is 3/4 of the total, giving KE:PE = 3:1. A The energies are equal only when x = A/√2, not at x = A/2, so KE ≠ PE here. B The speed is not linearly related to displacement; v/v_max = √(1 − (x/A)^2) = √(1 − 1/4) = √3/2 ≈ 0.87, not 0.5. C This matches the energy partition at x = A/2, so it is correct. D The restoring force is not zero at x = A/2; it is proportional to displacement and is half its maximum magnitude, not zero.

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