2ah=Vt^2-V0^2 V0=0 a=8m/s2 得Vt^2=480 入水前动能 0.5MVt^2=240M
设入水后下潜x, 由动能定理得 0-0.5MVt^2=Mg*x-3.5Mg*x 解得x=9.6米