f.4 phys..

用戶97935

2008-10-07 7:35 pm

On a straight horizontal road, a car decelerates uniformly to rest. During the deceleration, the speed of the car decreases from 30kmh^1 to 15kmh^1 after travelling a distance of s. How much farther will the car travel before it comes to rest?

a. 1/3 s

b. 1/2 s

c. s

d. 2s

回答 (1)

Audrey Hepburn

2008-10-07 7:45 pm

✔ 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

參考: Myself~~~

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