( ( When ( ( one object ) ( ( ( exerts ( ( a force ) ( on another ) ) ) , ) ( according ( to ( ( ( Newton 's ) ( third law ) ) ( of motion ) ) ) ) ) ) ) ( , ( ( the ( second object ) ) ( ( will ( exert ( an ( opposite direction ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (CD one) (NN object)) (VP (VBZ exerts) (NP (NP (DT a) (NN force)) (PP (IN on) (NP (DT another)))) (, ,) (PP (VBG according) (PP (TO to) (NP (NP (NP (NNP Newton) (POS 's)) (JJ third) (NN law)) (PP (IN of) (NP (NN motion))))))))) (, ,) (NP (DT the) (JJ second) (NN object)) (VP (MD will) (VP (VB exert) (NP (DT an) (JJ opposite) (NN direction)))) (. .)))
When one object exerts a force on another, according to Newton's third law of motion, the second object will exert an opposite direction.
(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))))))))))) (. .)))