OK peeps, just trying to pull my thoughts together here... I'm going to sum
up which (I think!) has been said so far, as I understand it, so folks can
tell me if I've got it right!

Motion constrained to the X-Y plane:
  * Compute the resultant force acting on the particle as normal.
  * Set the Z-part to zero.
  * Compute motion as normal.

Motion constrained to a sphere:
  * Compute resultant force.
  * Constrain the force to a plane perpendicular to the string tethering the
particle.
  * Compute motion as normal.
  * Work out how far the particle has moved.
  * Rotate the particle from it's initial position so that it moves the same
amount.

About that last step... I know how to work out the distance the particle has
moved... How do I work out how far to rotate? Angle = distance / radius or
something?

Andrew.

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