Two blocks standing on oneanother are pulled with a force F. There's friction between them. With a certain force F the purple block is standing still on the green one (situation 1) because of static friction. Is the netto force working on the green block F+ static friction of the purple block (because Newtons third law implies the static friction on the purple block= the opposite static friction of the purple block on the green block)? And in the second situation the static friction is overcome and replaced by kinetic friction. Is the netto force working on the green block now F- kinetic friction of the purple block (again thanks to Newtons third law)?