( ( ( remember gravity ) ( if ( ( ( ( ( a force ) , ) and ) ( this force ) ) ( causes ( things ( to accelerate ) ) ) ) ) ) . )
(ROOT (S (VP (VB remember) (NP (NN gravity)) (SBAR (IN if) (S (NP (NP (DT a) (NN force)) (, ,) (CC and) (NP (DT this) (NN force))) (VP (VBZ causes) (S (NP (NNS things)) (VP (TO to) (VP (VB accelerate)))))))) (. .)))
remember gravity if a force, and this force causes things to accelerate.
(ROOT (S (NP (NNP Gravity)) (VP (VBZ causes) (S (NP (NNS rocks)) (VP (TO to) (VP (VB roll) (ADVP (RB downhill)))))) (. .)))