[ODE] Heavy objects sinking

[NoWay InHell]

The reaction forces being calculated are insufficient for the given
density/mass value you have chosen.  Think of the combination of your
ERP/CFM values as defining a spring constant.  The amount of force
being applied by your force field on this mass is greater than the
spring can exert in response.  Changing those values will help solve
your problem, but will increase the pure-rigid/100% elasticity of the
response calculations.

 - Zink