( ( 1 ( ( Static and ) ( Sliding Friction ) ) ) . )
(ROOT (NP (NP (CD 1)) (NP (NP (NNP Static)) (CC and) (NP (NNP Sliding) (NNP Friction))) (. .)))
1 Static and 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)))))))))))))) (. .)))