物理問題--Kinematic

2007-09-14 3:17 pm

Winnie leaves A and Mary leaves B at the same time, both heading for the other's city along identical routes.They both have constant speeds(but, of couse,Mart is faster),and they pass at noon without stopping.If Mary bikes into A at 1:00PM and Winnie hikes into B at 2:15PM, at what time did they both set off?

我諗黎諗去都諗唔到點計，未知數有太多 = =

回答 (1)

Carson

2007-09-14 4:04 pm

✔ 最佳答案

Let Winnie and Mary set off at x hours before noon
and distance between A and B is d km
Mary biked for x + 1 hr
Winnie biked for x + 2.25 hr
speed for Mary = d / (x + 1) km/hr
speed for Winnie = d / (x + 2.25) km/hr
Before meeting, distance travelled by Winnie + distance travelled by Mary = d
x [ d / (x + 1) ] + x [ d / (x + 2.25) ] = d
x / (x + 1) + x / (x + 2.25) = 1
x / (x + 2.25) = 1 - x / (x + 1)
x / (x + 2.25) = 1/ (x + 1)
x * (x + 1) = (x + 2.25)
x^2 + x - x = 2.25
x^2 = 2.25
x = 1.5
they both set off at 10:30am (because 12 - 1.5 = 10.5)
I hope this can help your understanding. =)

參考: me ^^

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