( ( ( ( ( ( ( ( When ( ( two objects ) interact ) ) ( , ( ( ( the forces ) ( they ( exert ( on ( each other ) ) ) ) ) ( are equal ) ) ) ) and ) ( ( ( opposite and ) ( these forces ) ) ( act ( for ( the ( same time ) ) ) ) ) ) , ) so ) ( ( the impulses ) ( are ( ( equal and ) opposite ) ) ) ) . )
(ROOT (S (S (S (SBAR (WHADVP (WRB When)) (FRAG (ADJP (NP (CD two) (NNS objects)) (JJ interact)))) (, ,) (NP (NP (DT the) (NNS forces)) (SBAR (S (NP (PRP they)) (VP (VBP exert) (PP (IN on) (NP (DT each) (JJ other))))))) (VP (VBP are) (ADJP (JJ equal)))) (CC and) (S (NP (NP (NN opposite)) (CC and) (NP (DT these) (NNS forces))) (VP (VBP act) (PP (IN for) (NP (DT the) (JJ same) (NN time)))))) (, ,) (IN so) (S (NP (DT the) (NNS impulses)) (VP (VBP are) (ADJP (JJ equal) (CC and) (JJ opposite)))) (. .)))
When two objects interact, the forces they exert on each other are equal and opposite and these forces act for the same time, so the impulses are equal and opposite.
(ROOT (S (NP (NP (NNS Objects)) (PP (IN on) (NP (NN earth)))) (VP (VBP have) (NP (NP (CD two) (NNS forces)) (VP (VBG acting) (PP (IN on) (NP (PRP them))) (PP (IN at) (NP (DT all) (NNS times)))))) (. .)))