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)? http://img363.imageshack.us/img363/3448/situations9qp.gif [Broken] 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)?