AS Daily A Level Physics question
A car on a dry road brakes with a steady deceleration. From 15 m/s it takes 20 m to come to rest. Under the same conditions, what is the stopping distance from 30 m/s (ignore reaction time)?
Answer
The correct answer is A.
Correct: A — 80 m. With the same constant deceleration, stopping distance is proportional to the square of the initial speed, so doubling speed makes distance ×4: 4 × 20 m = 80 m. A … B — 40 m assumes distance is proportional to speed (doubling speed doubles distance), but the distance scales with speed squared, not linearly. C — 20 m implies stopping distance is independent of initial speed, which is false. D — 60 m reflects a mix-up between time and distance; while stopping time doubles when speed doubles, the distance increases by a factor of four, not three.