✔ 最佳答案
Consider the first journey
Initial speed, u = 30 kmh-1 = 8.33 ms-1
Final speed, v = 15 kmh-1 = 4.17 ms-1
Travelling distance = s
By v2 = u2 + 2as
(4.17)2 = (8.33)2 + 2as
Acceleration, a = -26.04 / s
Now, consider the second half of the journey
Initial speed, u = 4.17 ms-1
Final speed, v = 0
Acceleration, a = -26.04 / s
By v2 = u2 + 2ad
0 = (4.17)2 + 2(-26.04 / s)d
d = 1/3 s
So, the answer is A.
2008-10-07 12:40:53 補充:
0 = (4.17)2 + 2(-26.04 / s)d <<<呢個,,
d = 1/3 s<<<點計到呢個
唔好意思數學唔係錦好.""""""
0 = (4.17)2 + 2(-26.04 / s)d
-52.08d / s = -17.36
d / s = 1/3
d = 1/3 s