( That ( ( means ( that ( ( ( the force ) ( of ( gravity downwards ) ) ) ( must ( be ( balanced ( by ( the ( magnetic ( force upwards ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT That)) (VP (VBZ means) (SBAR (IN that) (S (NP (NP (DT the) (NN force)) (PP (IN of) (NP (NN gravity) (NNS downwards)))) (VP (MD must) (VP (VB be) (VP (VBN balanced) (PP (IN by) (NP (DT the) (JJ magnetic) (NN force) (NNS upwards))))))))) (. .)))
That means that the force of gravity downwards must be balanced by the magnetic force upwards.
(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.