BGA Example bga-7 for the Surface Evolver

This model introduces lateral displacement of the upper pad, with calculations of lateral and vertical forces. Otherwise, it is the same as bga-3.fe.

Notable features:

• The lateral displacement is implemented by a new parameter, "offset", which is used in constraint 4, the rim of the upper pad.
• The file "xzforce.cmd" contains the force calculation commands, "do_xforce" and "do_zforce". These use the central difference, linear interpolation method.
• The initial display may have the displacement coming out of the screen so you can't see it. Just rotate the image.
• Notice that changing the offset does not keep even spacing of the vertices on the top pad, since vertices just move to satisfy the constraints, not to keep even spacing. One cure for this is in bga-8.fe.