m*v*L/2 = 0 + 1/3 M*L^2*ω, 1/2 (1/3 M*L^2)*ω^2 = M*g*L/2*(1 - cosq)联立解出 v = (2 M √[Lg(1-cosq)] / (m√3)
动量矩=角动量守恒吧。