AS Daily A Level Physics question
On a dry test track a car brakes uniformly from 20 m s^-1 to rest in 25 m. Under the same conditions with the same braking force, the driver repeats the test starting at 30 m s^-1. Which is the best estimate of the stopping distance?
Answer
The correct answer is B.
Correct: B — 56 m. With the same braking force (constant deceleration), stopping distance is proportional to the square of the initial speed, so s2 = 25 × (30/20)^2 = 25 × 2.25 ≈ 56 m. A assumes linear scaling with speed (25 × 1.5), ignoring the square dependence. B is correct as shown. C overestimates by assuming a stronger-than-square scaling or confusing distance with time. D wrongly implies stopping distance is independent of initial speed, ignoring the greater initial kinetic energy that must be removed.