( ( An ( elastic collision ) ) ( ( occurs ( when ( ( a ball ) ( collides ( with ( ( ( the ( other ball ) ) or ) ( the ground ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT An) (JJ elastic) (NN collision)) (VP (VBZ occurs) (SBAR (WHADVP (WRB when)) (S (NP (DT a) (NN ball)) (VP (VBZ collides) (PP (IN with) (NP (NP (DT the) (JJ other) (NN ball)) (CC or) (NP (DT the) (NN ground)))))))) (. .)))
An elastic collision occurs when a ball collides with the other ball or the ground.
(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))))) (. .)))