physics (vertical motion under gravity)

Ok. let me answer your question step by step. Assume upwards means negative and assume g=10 for simplicity.

Step One - stone going up
if the stone reaches its highest point, the speed becomes zero
v^2=u^2+2as, where v=0; a=+g; u=-16

Hence, s = -12.8m (negative means moving upwards; this value is measured from the top of the building to the highest point of the stone can reached)

Also, v=u+at, where v=0; u=-16; a=+g

Hence, t=1.6seconds (time requried for the stone going up and reaching the highest point)

Step two - stone going down
at the highest point, the speed becomes zero; i.e. the initial speed is zero.
s=ut+0.5*at^2, where u=0; a=+g; t= 4-1.6=2.4seconds

Hence, s=0.5*10*2.4^2=28.8m (this value is the distance from the highest point to the ground)

Step three
The height of the building = 28.8-12.8=16m

***其實仲有更加簡單的方法，不過如果你concept弱既話，你可以參考我果個較長既答案先，明白左之後再行捷徑。我打左好耐，希望幫到你

2007-02-15 13:53:51 補充：
較詳細既答案可以參考上面果位人兄一條式一次過，點解可以咁呢？其實係因為站o係displacement既立場，石頭上拋既displacement會於下落既時候被抵銷。你細心畫圖諗下

take upward direction as negative
By s = ut + 0.5at^2
s = (-16)(4) + 0.5g(4)^2
s = -64 + 0.5*10*16 = 16m

2007-02-15 12:40:41 補充：
ie height of the building = 16m

2007-02-15 18:27:37 補充：
Thanks, the guy belowActually my good concepts come from HL Tam of 會考教室If you find that you are not well in Physics, you could find him.