Back to Daily Question archive

AS Daily A Level Physics question

2026-06-05 OCR A Mechanics & Materials (M3) OCR-A Module 3.3.1 Work and conservation of energy

On a dry test track the same car makes two emergency stops using the same brakes so the braking force is effectively constant. From 15 m/s it comes to rest in 20 m (braking distance only). From 30 m/s, what is the braking distance, and why?

  1. A 20 m, because friction sets the distance and initial speed doesn’t matter.
  2. B 40 m, because doubling the speed doubles the distance when the force is the same.
  3. C 60 m, because the time to stop doubles and the average speed during braking is roughly the initial speed.
  4. D 80 m, because the energy to remove increases with the square of speed while the braking force is the same. (correct)

Answer

The correct answer is D.

Correct: D — 80 m, because the energy to remove increases with the square of speed while the braking force is the same. The kinetic energy scales with v^2, so doubling speed makes energy 4×; with the same braking force, distance scales 4×: (30/15)^2 × 20 m = 80 m. A — Incorrect: braking distance does depend on initial speed because more kinetic energy must be removed at higher speed. B — Incorrect: this treats distance as proportional to speed, ignoring that kinetic energy scales with speed squared. C — Incorrect: it mixes up time and distance; while stopping time doubles for constant deceleration, the distance does not triple and must match the v^2 energy change. D — Correct as shown by the v^2 energy scaling and constant braking force implying distance ∝ energy/force.

Try today's guest Daily MCQ A Level Physics MCQ practice Problem-solving previews