→ denisck123:我忘了考慮時間問題 如果t<6 則 7.2(t-6)這項不見 05/06 22:11
※ 引述《cutiehoneyzu (McHilo)》之銘言：
: [領域] 運動學
: [來源] vector mechanics for engineers 7th
: [題目]11.74 An elevator starts from rest and moves upward.accelerating at a
: rateof 1.2m/s^2 until it reach a speed of 7.2m/s ,which it then maintains.
: two seconds after the elevator begins to move ,a man standing 12m above
: the initial position of the top of the elevator throws a ball upward
: with an initial velocity 20m/s.Determine when the ball will hit the elevator
: ^ 20m/s
: o |
: |--o ____
: _ _ _ ^_
: | |
: | |
: |a=1.2| 12m
: | ^ |
: | | |
: |_ _ _| ____
: |eleva|tor
: |_ _ _|
: [瓶頸] 最困難的是在電梯向上加速後又變成等速上升
: y=v0t+0.5a t^2 又用不得因為它後來變為等速上升
: 而且人在2秒後向上拋出一球就變得很困難
(7.2)/(1.2)=6
elevator y= 0.5(1.2)t^2 + 7.2(t-6)
ball y'=12 + 20(t-2) - 0.5(10)(t-2)^2
y=y'
應該是這樣吧@@
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