( Friction ( ( acts ( to ( ( keep ( an object ) ) ( from ( ( sliding or ) moving ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Friction)) (VP (VBZ acts) (S (VP (TO to) (VP (VB keep) (NP (DT an) (NN object)) (PP (IN from) (S (VP (VBG sliding) (CC or) (VBG moving)))))))) (. .)))
Friction acts to keep an object from sliding or moving.
(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)))))))))))))) (. .)))