s = v3t + 1/2at^2
t = 1s,a = -g = -10m/s^2,s = 0m => v3=5m/s(第三秒时的速度)
v0=v3+2g = 25m/s (上抛时的速度)
总时间为:t总 = 2*v0/g = 5s