[ODE] Best means of determing the strength of a contact?

[Megan Fox]

By that, I mean determining the force of a contact between two objects, for
the purposes of deciding what sound effect to play (or how much damage to
inflict, etc).

For the moment, the best way seems to be to project the velocity vector onto
the contact normal and take the length of the projected vector... but is
there a better way?  Will this way give me unstable results?

Thanks,

-Megan Fox