The rock with mass 400 kg is placed on the ground and the coefficient of friction between the rock and the ground is 0.8.

The resistive force of friction between the ground and the rock is equal to `N*mu` where `mu` is the coefficient of friction and N is the normal force. Here `mu = 0.8` and N = 400*9.8 = 3920 N. This gives the resistive force due to friction as 3136 N

If the rock is pulled with the minimum amount of force that can be used, the work required to be done to pull the stone by 400 m is 3136*400 = 1254400 J or 1254.4 kJ

To move the rock by 400 m the work required to be done is equal to 1254.4 kJ