( However ( , ( ( because ( ( its direction ) ( of ( ( motion changes ) ( on impact ) ) ) ) ) ( , ( ( a ( resultant force ) ) ( ( must ( have ( been ( exerted ( by ( ( the ( solid surface ) ) ( on ( the ( gas molecule ) ) ) ) ) ) ) ) ) . ) ) ) ) ) )
(ROOT (S (ADVP (RB However)) (, ,) (PP (IN because) (NP (NP (PRP$ its) (NN direction)) (PP (IN of) (NP (NP (NN motion) (NNS changes)) (PP (IN on) (NP (NN impact))))))) (, ,) (NP (DT a) (NN resultant) (NN force)) (VP (MD must) (VP (VB have) (VP (VBN been) (VP (VBN exerted) (PP (IN by) (NP (NP (DT the) (JJ solid) (NN surface)) (PP (IN on) (NP (DT the) (NN gas) (NN molecule))))))))) (. .)))
However, because its direction of motion changes on impact, a resultant force must have been exerted by the solid surface on the gas molecule.
(ROOT (S (NP (NNP Work)) (VP (VBZ is) (VP (VBN done) (SBAR (RB only) (IN if) (S (NP (DT a) (NN force)) (VP (VBZ is) (VP (VBN exerted) (PP (IN in) (NP (NP (DT the) (NN direction)) (PP (IN of) (NP (NN motion))))))))))) (. .)))