( Sliding ( Friction ( Rolling ( Friction ( Fluid Friction ) ) ) ) )
(ROOT (NP (NNP Sliding) (NNP Friction) (NNP Rolling) (NNP Friction) (NNP Fluid) (NNP Friction)))
Sliding Friction Rolling Friction Fluid 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)))))))))))))) (. .)))