( A ( ( smile ( his ( centripetal force ) ) ) . ) )
(ROOT (S (NP (NNP A)) (VP (VBP smile) (NP (PRP$ his) (JJ centripetal) (NN force))) (. .)))
A smile his centripetal force.
(ROOT (S (NP (JJ Centripetal) (NN force)) (VP (VBZ is) (NP (NP (DT the) (NN force)) (SBAR (WHNP (WDT that)) (S (VP (VBZ causes) (NP (JJ centripetal) (NN acceleration))))))) (. .)))