( Static ( Friction ( Vs ( Sliding Friction ) ) ) )
(ROOT (NP (NNP Static) (NNP Friction) (NNP Vs) (NNP Sliding) (NNP Friction)))
Static Friction Vs Sliding Friction
(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)))))))))))))) (. .)))