记张力为T，加速度为a。两个方程：T*R = I*a/R; m*g-T = m*a.解得a = g * 2m/(2m+M) = g*16/31 =5.058 m/s^2; T = g*M*m/(2m+M) = g*(120kg)/31 =37.935 N;下落距离 h = 0.5*a*t^2 = g * (8/31) * 25s^2 = 63.226 m.
