( Force ( ( is ( ( ( a ( key concept ) ) , ) ( ( of which ) ( ( ( ( ( ( gravity , ) ( ( electricity and ) magnetism ) ) , ) and ) ( the ( nuclear force ) ) ) ( are examples ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Force)) (VP (VBZ is) (NP (NP (DT a) (JJ key) (NN concept)) (, ,) (SBAR (WHPP (IN of) (WHNP (WDT which))) (S (NP (NP (NN gravity)) (, ,) (NP (NN electricity) (CC and) (NN magnetism)) (, ,) (CC and) (NP (DT the) (JJ nuclear) (NN force))) (VP (VBP are) (NP (NNS examples))))))) (. .)))
Force is a key concept, of which gravity, electricity and magnetism, and the nuclear force are examples.
(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.