( ( The force ) ( of friction ) )
(ROOT (NP (NP (DT The) (NN force)) (PP (IN of) (NP (NN friction)))))
The force of friction
(ROOT (S (NP (NN Friction) (NN force)) (VP (VBZ is) (VP (VBN caused) (PP (IN by) (NP (NP (JJ microscopic) (NNS bumps)) (, ,) (VP (VBZ lumps)) (, ,) (CC and) (NP (NP (NNS imperfections)) (VP (VBG colliding))))))) (. .)))