( ( Internal energy ) ( ( ( is simply ) ( ( the sum ) ( of ( ( the ( kinetic ( and ( potential energies ) ) ) ) ( of ( ( all ( the molecules ) ) ( of ( the system ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Internal) (NN energy)) (VP (VBZ is) (ADVP (RB simply)) (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 (PDT all) (DT the) (NNS molecules)) (PP (IN of) (NP (DT the) (NN system))))))))) (. .)))
Internal energy is simply the sum of the kinetic and potential energies of all the molecules of the system.
(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)))) (. .)))