/* Computes the momentum exchange for an elastic collision.  One of the objects is at (0, 0) with velocity (0, 0). */
float []elasticCollision(float x, float y, float momx, float momy, float kick)
{
  float []ret = new float[2];
  float d = dist(0, 0, x, y);
  final float momInward = - (momx * x + momy * y) / d;
  if (momInward <= 0) return ret;
  ret[0] = (kick + momInward) * x / d;
  ret[1] = (kick + momInward) * y / d;
  return ret;
}