( that ( ( has ( to ( ( equal ( ( the force ) ( of gravity ) ) ) ( because ( ( ( the force ) ( of gravity ) ) ( is ( what ( is ( causing that ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT that)) (VP (VBZ has) (S (VP (TO to) (VP (VB equal) (NP (NP (DT the) (NN force)) (PP (IN of) (NP (NN gravity)))) (SBAR (IN because) (S (NP (NP (DT the) (NN force)) (PP (IN of) (NP (NN gravity)))) (VP (VBZ is) (SBAR (WHNP (WP what)) (S (VP (VBZ is) (VP (VBG causing) (NP (DT that))))))))))))) (. .)))
that has to equal the force of gravity because the force of gravity is what is causing that.
(ROOT (S (NP (NNP Gravity)) (VP (VBZ causes) (S (NP (NNS rocks)) (VP (TO to) (VP (VB roll) (ADVP (RB downhill)))))) (. .)))