设球回到出发点时的速度:V
1/2mv^2-1/2mv0^2= - f*2h
v=√(v0^2-4f*h/m)消去f0-1/2mv0^2=- fh-mgh f=mv0^2/2h-mg 带入 v=√(v0^2-4f*h/m) =√(4gh-v0^2)