Two blocks of masses 2 kg and 4kg are connected through a massless inextensible string. The co-efficient of fricition between 2kg block and ground is 0.4 and the coefficient of friction between 4kg block and ground is 0.6. The forces `F_(1)=10N` and `F_(2)=20N` are applied on the blocks as shown in the figure. Calculate the frictional force between 4 kg block and ground (Assume initially the tension in the string was just before forces `F_(1)` and `F_(2)` wer applied).