A block of mass m = 1 kg moving on a horizontal surface with speed vi=2m-1 enters a rough patch ranging from x = 0.10m to x = 2.01m. The retarding force Fr=-kx for 0.1m < x < 2.01 m, Fr=0 for x < 0 .1m and x >2.01 m where k = 0.5 J, the final speed uf of the block as it crosses this patch