( Weight ( ( is ( ( a force ) ( caused ( by gravity ) ) ) ) . ) )
(ROOT (S (NP (NNP Weight)) (VP (VBZ is) (NP (NP (DT a) (NN force)) (VP (VBN caused) (PP (IN by) (NP (NN gravity)))))) (. .)))
Weight is a force caused by gravity.
(ROOT (S (NP (NNP Gravity)) (VP (VBZ causes) (S (NP (NNS rocks)) (VP (TO to) (VP (VB roll) (ADVP (RB downhill)))))) (. .)))