( Friction ( ( is ( ( one force ) ( causes ( a ( ball ( to ( roll downhill ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Friction)) (VP (VBZ is) (NP (NP (CD one) (NN force)) (SBAR (S (VP (VBZ causes) (NP (DT a) (NN ball) (S (VP (TO to) (VP (VB roll) (ADVP (RB downhill))))))))))) (. .)))
Friction is one force causes a ball to roll downhill .
(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))))))) (. .)))