( ( ( Sliding ( ( friction , ) ( in contrast ) ) ) ( to ( static friction ) ) ) . )
(ROOT (S (VP (VBG Sliding) (NP (NP (NN friction)) (, ,) (PP (IN in) (NP (NN contrast)))) (PP (TO to) (NP (JJ static) (NN friction)))) (. .)))
Sliding friction, in contrast to static 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)))))))))))))) (. .)))