一质点做加速度为g的自由落体运动，在落地前的最后一秒内通过的距离为s，由此可求出确定的物理量为（ ）
A．质点落地前瞬间的速度
B．质点开始下落时距地面的高度
C．质点下落到距地面还剩一半高度时的速度
D．质点速度为落地前瞬间的速度一半时的高度
设自由落体运动的总时间为t，则最后一秒内的位移为s=
gt2-1 2
g(t-1)21 2
解得：t=
+s g 1 2
A、根据v=gt可以求出质点落地前瞬间的速度，故A正确；
B、根据h=
gt2可以求出质点开始下落时距地面的高度，故B正确；1 2
C、根据2g
=v2可以求出质点下落到距地面还剩一半高度时的速度，故C正确；h 2
D、根据2gh′=(
)2可以求出速度为落地前瞬间的速度一半时的高度，故D正确．gt 2
故选ABCD
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