初速度v0,斜面高度h,斜面中间高度h/2,斜面中间速度v
根据能量守恒：
mgh = 1/2mv0^2 = mgh/2+1/2mv^2
h=v0^2/(2g)
1/2mv0^2 = mg*[v0^2/(2g)]/2+1/2mv^2 = 1/4mv0^2+1/2mv^2
1/2v^2=1/4v0^2
v^2 = 1/2v0^2
v = √2/2 v0