( we ( thus ( ( speak ( of ( ( the force ) ( of ( ( ( ( electricity , ) ( of magnetism ) ) , ) ( of gravity ) ) ) ) ) ) . ) ) )
(ROOT (S (NP (PRP we)) (ADVP (RB thus)) (VP (VBP speak) (PP (IN of) (NP (NP (DT the) (NN force)) (PP (IN of) (NP (NP (NP (NN electricity)) (, ,) (PP (IN of) (NP (NN magnetism))) (, ,)) (PP (IN of) (NP (NN gravity)))))))) (. .)))
we thus speak of the force of electricity, of magnetism, of gravity.
(ROOT (S (NP (NNP Gravity) (CC and) (NNP magnetism)) (VP (VBP are) (NP (DT both) (JJ common) (NNS forces))) (. .)))
Gravity and magnetism are both common forces.