落地时速度方向与水平方向的夹角为a,即：
vy/vx=tanα
vy=vxtanα=v0tanα
h=vy^2/(2g)=(v0tanα)^2/(2g)
重力势能和动能之比:
mgh/(1/2mvo^2)=2gh/v0^2=2g[(v0tanα)^2/(2g)]/v0^2=(tanα)^2