f.4 phys...

2008-10-08 2:11 am

On a straight horizontal road, Sam is driving at a uniform speed of 18ms^-1. Suddenly he observes an obstacle 35m ahead. He quickly applies the brake and the car is braked to stop in 3s. Suppose his reaction time is 0.2s.

a) How far in front of the obstacle has the car been stopped?

b) Explain whether the car can stop in time if its initial speed is 20ms^-1. Assume Sam's reaction time and the deceleration of the car remain unchanged.

回答 (1)

Audrey Hepburn

2008-10-08 4:01 am

✔ 最佳答案

a. Thinking distance = 0.2 X 18 = 3.6 m

For braking distance, apply s = (u + v)t / 2

s = (18 + 0)(3)/2 = 27 m

So, total stopping distance = 27 + 3.6 = 30.6 m

Hence, the car is stopped 35 - 30.6 = 4.4 m in front of the obstacle.

b. Thinking distance = 20 X 0.2 = 4 m

For part a, apply v = u + at

0 = 18 + a(3)

Acceleration, a = -6 ms-2

Now, apply v2 = u2 + 2as

0 = (20)2 + 2(-6)s

Braking distance, s = 33.33 m

Hence, total stopping distance = 33.33 + 4 = 37.33 m > 35 m

Hence, the car cannot stop in time.

2008-10-15 18:56:47 補充：
不可以，因為braking time t你是不知道的。

參考: Myself~~~

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