阻力 f = - k v^2x = c t^3 , v = 3 c t^2阻力做功Af = ∫ f dx = ∫ - k v^2 dx = ∫ - k v^2 dx/dt dt= ∫ - k v^3 dt = ∫ (0, (l/c)^(1/3)) - k (3 c t^2)^3 dt = - (27/7) c k l^2 (l/c)^(1/3)
