### Question

I know that gravity keeps us on the earth, but what prevents us from being pushed or pulled through the floor, earth, etc. (besides concrete, wood, and soil)? Does it have something to do with opposite forces?

In a nut shell, the answer is the electromagnetic field.

Until the beginning of this century, there were only two known force fields -- gravity and the electromagnetic (EM) field. The strong and weak nuclear fields were not known. The electromagnetic field manifests itself in many different ways. The force between magnets is a manifestation of the magnetic aspect of the EM field. The force that attracts your hair when you comb it in winter is a manifestation of the electric aspect of the EM field. Light, radio waves, gamma rays, etc. are manifestations of the simultaneous combination of alternating magnetic and electric fields. James Clerk Maxwell, in his famous 'Maxwell's equations' provided a proper mathematical basis for understanding the unification of the magnetic and electric aspects of the EM field.

When two ordinary objects, such as your shoes and the floor on which you stand, come close enough together, the electrons that surround the atoms of the floor and of your shoes repel each other strongly through the electic force (the so-called Coulomb force). Specifically, electrons in your shoes and in the floor strongly repel each other because they carry like (by convention, 'negative') electric charges, which are responsible for creating similar electric fields around each electron. The force of repulsion is mediated by the electric fields established by the charges on the electrons.

The magnitude of this force increases inversely as the distance between the sources of the field, the electrons. Hence, the electric forces can quickly become very great, even great enough to support the weight of your body against the attractive force of the other significant field -- gravity.

Because of the repulsive force of the electic field of the electrons, the nuclei of atoms do not come close enough to each other under ordinary circumstances for the effects of the two nuclear forces to be observable. Exceptions are in the interior of a star, a thermonuclear (H-bomb) explosion, and in the plasma in a Tokomak used to generate energy by thermonuclear fusion.

As you suggest, opposite forces are involved. But, not a force opposite to gravity. Rather, it is the opposite, or repulsive, force between like electric charges that reside in concrete, wood, soil, etc. that supports you against gravity, and, indeed, that enables us to interact with all ordinary objects in our environment.
Answered by: Warren Davis, Ph.D., President, Davis Associates, Inc., Newton, MA USA

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'You cannot hope to build a better world without improving the individuals. To that end each of us must work for his own improvement, and at the same time share a general responsibility for all humanity, our particular duty being to aid those to whom we think we can be most useful.'

Marie Curie
(1867-1934)