重力做的正功为 W1 = mgh
阻力做的负功为 W2 = - f L
初动能为零,末动能为 1/2 mv²
由动能定理可得：mgh - f L = 1/2 mv² - 0
解得：v = √2(mgh - f L)/m = √2(60×10×3.2 - 240×4)/60 m/s = 4√2 m/s = 12.56 m/s