( Then ( we ( ( are ( to ( calculate ( the ( elastic ( potential energy ) ) ) ) ) ) . ) ) )
(ROOT (S (RB Then) (NP (PRP we)) (VP (VBP are) (S (VP (TO to) (VP (VB calculate) (NP (DT the) (JJ elastic) (JJ potential) (NN energy)))))) (. .)))
Then we are to calculate the elastic potential energy.
(ROOT (S (S (VP (VBG Changing) (NP (NP (DT the) (NN shape)) (PP (IN of) (NP (DT an) (JJ elastic) (NN material)))))) (VP (VBZ gives) (NP (PRP it)) (NP (JJ potential) (NN energy))) (. .)))