解: (1)f=mg; m=f/g=ρv^2s/(2g)=3000ρs; (2)200m/s>v>8m/s时,a突然增大后减小方向向上,v减小 ; v=8m/s后a为零,v不变; (3)W=FS=mv^2/2=0.5*2700*8*8=86400; F=W/S=86400/1.2=72000;
