Two cyclists A and B are travelling in the same direction along a straight. B is travelling at a speed of 9m/s and A is overtaking B at a speed of 10m/s. At the moment when they are level they see a traffic light turn red 108m ahead. A cycles for T s and then decelerates uniformly;B cycles for 6s and then decelerates uniformly. A and B stop at the traffic light at the same instant.
a) Calculate the time during which B was decelerating
b) Calculate the value of T
[急]physics question(mechanics)
2009-09-24 8:21 am
回答 (1)
2009-09-24 3:04 pm
✔ 最佳答案
a. Travelling distance when B is cycling uniformly, d = 9 X 6 = 54 mSo, the distance to decelerate, s = 108 - 54 = 54 m
By s = (u + v)t / 2
54 = (9 + 0)t / 2
Time, t = 12 s
b. By v = u + at
0 = 9 + a(12)
Acceleration, a = -0.75 ms-2
Consider A, by v2 = u2 + 2as
0 = (10)2 + 2(-0.75)s
Decelerating distance, s = 66.67 m
So, the thinking distance, d = 108 - 66.67 = 41.33 m
Therefore, T = d / v = 41.33 / 10 = 4.13 s
參考: Physics king
收錄日期: 2021-04-19 15:26:16
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