( Friction ( ( is ( ( a force ) ( that ( ( holds back ) ( ( the movement ) ( of ( a ( sliding object ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Friction)) (VP (VBZ is) (NP (NP (DT a) (NN force)) (SBAR (WHNP (WDT that)) (S (VP (VBZ holds) (PRT (RP back)) (NP (NP (DT the) (NN movement)) (PP (IN of) (NP (DT a) (JJ sliding) (NN object))))))))) (. .)))
Friction is a force that holds back the movement of a sliding object.
(ROOT (S (NP (VBG Sliding) (NN friction)) (VP (VBZ is) (NP (NP (NN friction)) (SBAR (WHNP (WDT that)) (S (VP (VBZ acts) (PP (IN on) (NP (NP (NNS objects)) (SBAR (WHADVP (WRB when)) (S (NP (PRP they)) (VP (VBP are) (VP (VBG sliding) (PP (IN over) (NP (DT a) (NN surface)))))))))))))) (. .)))