( ( forces ( from ( ( the ( Earth 's ) ) gravity ) ) ) ( probably ( ( cause this ) . ) ) )
(ROOT (S (NP (NP (NNS forces)) (PP (IN from) (NP (NP (DT the) (NN Earth) (POS 's)) (NN gravity)))) (ADVP (RB probably)) (VP (VB cause) (NP (DT this))) (. .)))
forces from the Earth's gravity probably cause this.
(ROOT (S (NP (NNP Gravity)) (VP (VBZ causes) (S (NP (NNS rocks)) (VP (TO to) (VP (VB roll) (ADVP (RB downhill)))))) (. .)))