1. Nature tends toward equilibrium - or lowest possible energy state.
2. Gravity is mutual - two masses attract each other - equal and opposite. As Gokul corrected me, "Force is a binary operator, F = f(m1,m2). The field due to a more massive object is greater. The force exerted by the earth on the moon equals the force exerted by the moon on earth - Newton's Third." Electric/magnetic forces are also mutual - attraction between opposite charges/poles and repulsion between like charges/poles.
briton, as for the earth/stone system, to what electrostatic force do you refer?
Going to the atomic level, the electron fields (and the atomic bonds they form) around the nuclei maintain spacing between nuclei/atoms. Solids are relatively incompressible - i.e. low strains at moderate pressures/compressive stresses (MPa range) - but at high pressures (GPa) interatomic spacing will decrease substantially.
As for measuring atomic forces, this can be done by atomic force microprobes, which have been developed by companies such as IBM. As far as I know, AFM's are applied under vacuum conditions.
High pressures on materials in the laboratory can be applied by diamond anvils - see examples at - http://hannahsmac.magnet.fsu.edu/F_S/high_pressure.html [Broken]
As atoms get close, the potential keeps decreasing (a negative potential indicates an attractive force) till it reaches a minimum. As the separation decreases beyond this, the potential increases till it starts becoming positive (top left portion of curve). Now the force between the atoms is repulsive.
For larger atoms than hydrogen, the PE curve is more complex, with several alternating positive and negative regions. Each of these positive regions represents a repulsion. This repulsion is the basis of the reaction force.
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