|MadSci Network: Physics|
Ah, now I understand better. If you firmly attach all the coils to one chassis, there are no external forces acting on the whole system. No external forces means that the system will not move.
The internal forces cancel each other due to the principle of action and reaction. Compare this to sitting in a box and pushing the inner wall. The box will not move. In this case, while the Helmholz coils are pused upwards, the central coil is pushed downwards by an equal amount.
Hope this answers your question.
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