Look at line a,assume it is not sliding down.Line b is sliding down,which means the frictional force of floor is smaller than F. Line is of mass 1kg.Length is 2m, so C.M is in 1m How do you find the force F interms of L,Lh, Lv,m each case? L is the length of line. Lh is the horizontal distance. Lv is Vertical distance. This is not a Home work Question or Homework type question . I am just curious.