# Contact force, EM force and the loop

1. Nov 5, 2008

### Saw

The maximum I have found about contact force is very roughly this: it is nothing but electromagnetic force, meaning that, if two bodies collide, the electrons of the atoms of the outer shell of each body repel each other, those atoms displace towards their neighbours and repel them in turn, thus creating a sort of wave. My reflection is: if in the end, following Newton's Second Law, the body with more mass must get accelerated less and the body with less mass is to be accelerated more, this must be because the "waves" reach everywhere in each body, come back and come with information about how much mass the body has, so that the final result in terms of acceleration would depend on the relative difference between the two masses... But, if this speculation is close to the truth, why does the wave bounce back at all, if it does, and why does this process end at some time, having as outcome the acceleration of the bodies, and not continue in a loop of endless oscillation...?

Last edited: Nov 5, 2008
2. Nov 6, 2008

### Saw

Well, as I did not get any answer, I have started to correct my own question. There may not exist any need for the wave coming back. If body A has just one atom, the latter gets repelled with a given acceleration and that is it. If body B has a number of atoms, then its acceleration is opposed by the electrons of such atoms... (?). But surely there is no need for my speculations, since there must exist a standard scientifically recognised answer for the question. Can anyone help? Thanks.

3. Nov 6, 2008

### Andy Resnick

'contact forces', meaning a description of what happens when two bodies 'touch', is a concept from continuum mechanics. Although at the atomic level, contact forces arise from electrostatic repulsion, there is not yet a complete theory for contact forces- i.e. how stress and strain is transmitted from one body to another.

Note that this does not preclude modeling the process numerically; this requires constitutive relations that come from experiments.