40．

ρ_A= G_A/g V_A=5×10³ Kg/m³                                        （1分）

η_A=（G_A―F_浮）/2F_拉 =80%                                     （1分）

G_A=500N                                                       （1分） 

G_B=900N

G_动 =100 N

F浮=ρ_水gV_A=100N                                              （1分）

F_拉=1/2（G_A+G_动-F_浮）=250N

G_A+G_动=600N      ①

η_B=G_B/(G_B+G_动)

G_B/(G_B+G_动) =90% ②

G_A：G_B=5：9     ③

由①②③式可得

F_拉=G_人-F_支=600N-350N=250N                                     （1分）

39．F_支=F_压=8.75×10³ Pa×400×10^-4 m²=350N

      = 6.3×10⁵J                                                  (1分)

      = 4.2×10³J/ (kg?℃) ×3 kg×3 (95℃-45℃)                         (1分)

38．Q = cm (t-t₀)                                                  (1分)