( ( ( 5 -RRB- ) ( in ( the ( friction ( force loop ) ) ) ) ) ( ( are ( caused ( by ( the ( surface steps ) ) ) ) ) . ) )
(ROOT (S (NP (NP (CD 5) (NNS -RRB-)) (PP (IN in) (NP (DT the) (NN friction) (NN force) (NN loop)))) (VP (VBP are) (VP (VBN caused) (PP (IN by) (NP (DT the) (NN surface) (NNS steps))))) (. .)))
5) in the friction force loop are caused by the surface steps.
(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))))))) (. .)))