( This ( ( ( ( ( exerts force ) ( on ( the stomach ) ) ) , ) ( ( pushing ( its contents ) ) upward ) ) . ) )
(ROOT (S (NP (DT This)) (VP (VBZ exerts) (NP (NN force)) (PP (IN on) (NP (DT the) (NN stomach))) (, ,) (S (VP (VBG pushing) (NP (PRP$ its) (NNS contents)) (ADVP (RB upward))))) (. .)))
This exerts force on the stomach, pushing its contents upward.
(ROOT (S (NP (JJ Bouyant) (NN force)) (VP (VBZ is) (NP (NP (DT the) (JJ upward) (NN force)) (VP (VBN exerted) (PP (IN by) (NP (NP (NNS fluids)) (SBAR (WHADVP (WRB when)) (S (NP (NP (DT any) (NN object)) (, ,) (PP (JJ such) (IN as) (NP (DT a) (NN boat))) (, ,)) (VP (VBZ is) (VP (VBN placed) (PP (IN in) (NP (PRP it)))))))))))) (. .)))