montion and force

Use equation of motion: s = vt - (1/2)at^2
with s = 200 m, a = -g(=-10 m/s2), v = 0 m/s, t =?
hence, 200 = -(1/2)(-10)t^2
i.e. t = 6.325 s

Then use equation of motion: v = u + at
with v = 0 m/s, a = -10 m/s2, t = 6.325 s, u =?
hence, 0 = u + (-10)(6.325)
i.e. u = 63.25 m/s

The velocity of the firework is 6.325 m/s when it is fired.


2011-09-17 14:10:19 補充：
sorry, a typo in the naswer, it should read: "The velocity of the firework is 63.25 m/s when it is fired."

由於問題冇叫你去計t,所以唔使咁煩去計t先,可以直接用公式v^2=u^2+2as
以向上為正
v^2=u^2+2as 0=u^2-4000u^2=4000 u=63.25m/s
所以初速度係63.25m/s向上考試個陣要靈活運用個四條公式,清楚個四條公式唔需要D咩,如v^2=u^2+2as就唔需要知道時間t,係唔需要求時間ge題目入面,用E條就最啱.
如果唔係,浪費時間之餘,仲有更大機會計錯