PHYSICS 兩車相遇的問題

Wing Ho Winho

2009-09-13 3:55 pm

A car starts from rest bad maintains a uniform acceleration of 2.0m/s^2 along a straight road. As the car start, another car moving in the same direction along a parallel road with a constant velocity of 80km/hr passes it.
a. When will the cars be level with each other again?
b. What will be the velocity of the first car when the two cars are level?

一架車2.0m/s^2, 一架車80km/hr, 根據a part,
是同一個displacement而求time吧? 但一架提供acceleration, 一架提供velocity的資料, 兩個一起計都計不到的...

回答 (1)

Prof. Physics

2009-09-13 4:34 pm

✔ 最佳答案

Let s and t be the displacement of the cars and time elapsed for two cars to meet each other.

For the second car,

Speed, v = 80 km/h = 80/3.6 = 200/9 m/s

So, s = vt = 200t / 9 ... (1)

For the first car,

By s = 1/2 at2

200t / 9 = 1/2 (2.0)t2

t(t - 200/9) = 0

t = 0 (rejected) or 22.2 s

So, they will meet at time = 22.2 s.

For the first car,

By v = at

v = (2.0)(22.2)

Speed when they meet each other, v = 44.4 m/s

參考: Physics king

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