( ( First ( of all ) ) ( we ( ( have ( to ( remember ( that ( ( ( the ( two forces ) ) ( in ( ( an ( ( `` ( ( ( equal and ) opposite ) '' ) ) set ) ) ( of forces ) ) ) ) ( always ( act ( on ( TWO ( DIFFERENT OBJECTS ) ) ) ) ) ) ) ) ) ) . ) ) )
(ROOT (S (ADVP (JJ First) (PP (IN of) (NP (DT all)))) (NP (PRP we)) (VP (VBP have) (S (VP (TO to) (VP (VB remember) (SBAR (IN that) (S (NP (NP (DT the) (CD two) (NNS forces)) (PP (IN in) (NP (NP (DT an) (ADJP (`` ``) (JJ equal) (CC and) (JJ opposite) ('' '')) (NN set)) (PP (IN of) (NP (NNS forces)))))) (ADVP (RB always)) (VP (VB act) (PP (IN on) (NP (CD TWO) (JJ DIFFERENT) (NNS OBJECTS)))))))))) (. .)))
First of all we have to remember that the two forces in an &quot;equal and opposite&quot; set of forces always act on TWO DIFFERENT OBJECTS.
(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)))))) (. .)))