( ( Internal energy ) ( ( is ( ( ( ( the total ) ( of ( ( all ( the ( potential energy ) ) ) ( inside ( the molecules ) ) ) ) ) plus ) ( ( the ( kinetic energy ) ) ( of ( the molecules ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Internal) (NN energy)) (VP (VBZ is) (NP (NP (NP (DT the) (NN total)) (PP (IN of) (NP (NP (PDT all) (DT the) (JJ potential) (NN energy)) (PP (IN inside) (NP (DT the) (NNS molecules)))))) (CC plus) (NP (NP (DT the) (JJ kinetic) (NN energy)) (PP (IN of) (NP (DT the) (NNS molecules)))))) (. .)))
Internal energy is the total of all the potential energy inside the molecules plus the kinetic energy of the molecules.
(ROOT (S (NP (NP (DT The) (NN sum)) (PP (IN of) (NP (NP (DT the) (JJ kinetic) (CC and) (JJ potential) (NNS energies)) (PP (IN of) (NP (NP (DT a) (NN system) (POS 's)) (NNS atoms) (CC and) (NNS molecules)))))) (VP (VBZ is) (VP (VBN called) (NP (JJ internal) (NN energy)))) (. .)))