( It ( ( is ( ( this ( force ( opposing gravity ) ) ) ( that ( causes ( you ( to ( feel weight ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ is) (NP (NP (DT this) (NN force) (VBG opposing) (NN gravity)) (SBAR (WHNP (WDT that)) (S (VP (VBZ causes) (S (NP (PRP you)) (VP (TO to) (VP (VB feel) (NP (NN weight)))))))))) (. .)))
It is this force opposing gravity that causes you to feel weight.
(ROOT (S (NP (NNP Gravity)) (VP (VBZ causes) (S (NP (NNS rocks)) (VP (TO to) (VP (VB roll) (ADVP (RB downhill)))))) (. .)))