Masses
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  Accelerations on the masses are calculated according to gravity (and
other special forces), viscous forces, and by forces from the springs.
When a mass collides with a wall, its velocity in the direction of the
wall  is  reversed.   The  resulting  velocity  is  multiplied by  the
Elasticity of the mass.   So, an elasticity of 0.0 causes an inelastic
collision (it stops on the wall), and an  elasticity of 1.0 results in
an  elastic  collision.   If  a mass is  fixed, all  forces  on it are
ignored.  It  simply does  not move.  Think of  it as a nail (a really
good one).