( ( ( Sliding ( an object ) ) ( along ( ( the surface ) ( of ( another object ) ) ) ) ) ( ( results ( in ( sliding friction ) ) ) . ) )
(ROOT (S (S (VP (VBG Sliding) (NP (DT an) (NN object)) (PP (IN along) (NP (NP (DT the) (NN surface)) (PP (IN of) (NP (DT another) (NN object))))))) (VP (VBZ results) (PP (IN in) (S (VP (VBG sliding) (NP (NN friction)))))) (. .)))
Sliding an object along the surface of another object results in 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)))))))))))))) (. .)))