( ( ( The forces ) ( of gravity ) ) ( ( act ( upon ( ( ( both ( the flyer ) ) and ) ( the bases ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NNS forces)) (PP (IN of) (NP (NN gravity)))) (VP (VBP act) (PP (IN upon) (NP (NP (PDT both) (DT the) (NN flyer)) (CC and) (NP (DT the) (NNS bases))))) (. .)))
The forces of gravity act upon both the flyer and the bases.
(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.