A block needs 50 N of force to move and you are applying 5 N of force on that block in the horizontal direction. The frictional force acting on the block is
The total force needed to move the block is 50 N but, when you are applying 5 N of force on the block to make it move, the ground will apply same force which is just sufficient to oppose the motion of the block i.e. 5 N.