Two faces

6x4  4X5


Start in first face  3, 2
Stop in second face 3 1

midpoint in first face 6, 2.9
   ... in second face 0, 2.9
   
force vectors:
  (3, 2) - (6, 2.9) = (-3, -0.9)
  (3, 1) - (0, 2.9) = (3, -1.9)
  
sum vectors: (0, -2.8)

new midpoint: startpoint + coef (0.1)* sumvector
   (6, 2.9+0.1*(-2.8)) ~ (6, 2.62) -> new midpoint