( ( Physical ( Science ( Activation energy ) ) ) ( ( is ( ( the energy ) ( needed ( to ( start ( a ( chemical reaction ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Physical) (NNP Science) (NNP Activation) (NN energy)) (VP (VBZ is) (NP (NP (DT the) (NN energy)) (VP (VBN needed) (S (VP (TO to) (VP (VB start) (NP (DT a) (JJ chemical) (NN reaction)))))))) (. .)))
Physical Science Activation energy is the energy needed to start a chemical reaction.
(ROOT (S (NP (NP (DT The) (NN amount)) (PP (IN of) (NP (NP (NN energy)) (VP (VBN required) (S (VP (TO to) (VP (VB begin) (NP (DT a) (JJ chemical) (NN reaction))))))))) (VP (VBZ is) (VP (VBN known) (PP (IN as) (NP (DT the) (NN activation) (NN energy))))) (. .)))