A stone is dropped from a 75m high building.When this stone

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A stone is dropped from a 75m high building.When this stone has dropped 15m,a second stone is been dropped with an initial velocity such as two stone is been dropped with hit the ground at the same time.So what was the initial velocity of the second stone when it was droppe?
我算出来约等于十默默觉得错了.

题目意思是：一块石头从75米高的建筑下落,当这块石头下落15米时,第二块石头以某一初速度下落,这样,两块石头下落,并同时着地.那么,第二块石头下落时的初速度是多少?

先求出第一块石头下落第一段距离所用的时间：

根据h=gt²/2,得

t1=√(2h1/g)=√(2*15/10)s=√3s≈1.732s

再求出第一块石头全程所用的时间：

t=√(2h/g)=√(2*75/10)s=√15s≈3.873s

那么,第一块石头下落第二段距离所用的时间：

t2=t-t1=√15s-√3s=(√15-√3)s≈2.414s

那么,第二块石头下落的时间就和第一块石头在第二段距离下落的时间一样,则

h=gt²2/2+v0t2

v0=(h-gt²2/2)/t2=(75-10*2.414²/2)/2.414m/s≈21.57m/s

所以,第二块石头的初速度是21.57m/s.

【为了计算方便,g取10m/s²,根号值保留3为小数,结果保留2位小数】

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