( Sliding ( Friction Definition ) )
(ROOT (NP (NNP Sliding) (NNP Friction) (NNP Definition)))
Sliding Friction Definition
(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)))))))))))))) (. .)))