AS Daily A Level Physics question
A commuter train applies the same constant braking each time it approaches a platform. One day it begins braking at speed v and stops after distance s. On another day it begins braking at double the speed 2v, with the same constant deceleration throughout. Which statement must be true about the new stopping distance compared to s?
Answer
The correct answer is B.
Correct: B — It becomes four times larger. With the same constant deceleration, doubling the initial speed doubles the time to stop and also doubles the average speed during braking, so distance = (average speed) × (time) increases by 2 × 2 = 4. A is wrong because it assumes distance is only proportional to initial speed, ignoring that the stopping time also doubles. B is correct. C is wrong because a factor of three does not follow from the combined doubling of both average speed and stopping time. D is wrong because stopping distance depends on both the braking deceleration and the initial speed, not on deceleration alone.