( ( ( A force ) ( -LRB- ( ( ( friction force ) ( in ( these cases ) ) ) -RRB- ) ) ) ( ( must ( cause ( the slow-down ) ) ) . ) )
(ROOT (S (NP (NP (DT A) (NN force)) (PRN (-LRB- -LRB-) (NP (NP (NN friction) (NN force)) (PP (IN in) (NP (DT these) (NNS cases)))) (-RRB- -RRB-))) (VP (MD must) (VP (VB cause) (NP (DT the) (NN slow-down)))) (. .)))
A force (friction force in these cases) must cause the slow-down.
(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))))))) (. .)))