( ( ( The ( common form ) ) ( for ( ( gravity and ) ( the ( electric force ) ) ) ) ) ( ( ( comes ( from ( the fact ) ) ) ( that ( they ( both ( involve ( ( the exchange ) ( of ( ( ( a ( massless particle ) ) ( -LRB- ( ( ( ( ( the photon ) ( for ( the ( electric force ) ) ) ) and ) ( ( the graviton ) ( for gravity ) ) ) -RRB- ) ) ) ( which ( ( gives them ) ( ( ( infinite range ) and ) ( the ( common ( inverse ( square ( law form ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (JJ common) (NN form)) (PP (IN for) (NP (NP (NN gravity)) (CC and) (NP (DT the) (JJ electric) (NN force))))) (VP (VBZ comes) (PP (IN from) (NP (DT the) (NN fact))) (SBAR (IN that) (S (NP (PRP they)) (DT both) (VP (VBP involve) (NP (NP (DT the) (NN exchange)) (PP (IN of) (NP (NP (DT a) (JJ massless) (NN particle)) (PRN (-LRB- -LRB-) (NP (NP (NP (DT the) (NN photon)) (PP (IN for) (NP (DT the) (JJ electric) (NN force)))) (CC and) (NP (NP (DT the) (NN graviton)) (PP (IN for) (NP (NN gravity))))) (-RRB- -RRB-)) (SBAR (WHNP (WDT which)) (S (VP (VBZ gives) (NP (PRP them)) (NP (NP (JJ infinite) (NN range)) (CC and) (NP (DT the) (JJ common) (JJ inverse) (JJ square) (NN law) (NN form))))))))))))) (. .)))
The common form for gravity and the electric force comes from the fact that they both involve the exchange of a massless particle (the photon for the electric force and the graviton for gravity) which gives them infinite range and the common inverse square law form.
(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.