( ( Both ( elastic ( and ( charge ( exchange collisions ) ) ) ) ) ( ( must ( be considered ) ) . ) )
(ROOT (S (NP (DT Both) (NN elastic) (CC and) (NN charge) (NN exchange) (NNS collisions)) (VP (MD must) (VP (VB be) (VP (VBN considered)))) (. .)))
Both elastic and charge exchange collisions must be considered.
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (NN gas) (NNS particles)) (VP (VBP collide)))) (, ,) (NP (DT these) (NNS collisions)) (VP (VBP are) (VP (VBN considered) (S (ADJP (JJ elastic))))) (. .)))