( ( In ( this case ) ) ( ( the ( force ( you exert ) ) ) ( ( ( ( ( is ( in ( ( the ( opposite direction ) ) ( of ( the motion ) ) ) ) ) , ) and ) ( is ( considered ( to ( be negative ) ) ) ) ) . ) ) )
(ROOT (S (PP (IN In) (NP (DT this) (NN case))) (NP (DT the) (NN force) (NN you) (NN exert)) (VP (VP (VBZ is) (PP (IN in) (NP (NP (DT the) (JJ opposite) (NN direction)) (PP (IN of) (NP (DT the) (NN motion)))))) (, ,) (CC and) (VP (VBZ is) (VP (VBN considered) (S (VP (TO to) (VP (VB be) (ADJP (JJ negative)))))))) (. .)))
In this case the force you exert is in the opposite direction of the motion, and is considered to be negative.
(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))))))))))) (. .)))