( ( Rolling friction ) ( ( ( is not ) ( as ( straightforward ( as ( sliding friction ) ) ) ) ) . ) )
(ROOT (S (NP (JJ Rolling) (NN friction)) (VP (VBZ is) (RB not) (ADJP (RB as) (JJ straightforward) (PP (IN as) (S (VP (VBG sliding) (NP (NN friction))))))) (. .)))
Rolling friction is not as straightforward as 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)))))))))))))) (. .)))