f向下时Fx=8N;Fy=6N f=0.25*(mg+Fy)=6.5Na=(Fx-f)/m=0.75m^2/sv=at=7.5m/s2aS=v^2S=37.5mf向下时Fx=8N;Fy=6N f=0.25*(mg-Fy)=3.5Na=(Fx-f)/m=2.25m^2/sv=22.5m/s2aS=v^2S=112.5m