Physics - how to disable collision from some direction?

I am making a jump-game, like DoodleJump.
32813-collision_s.png

Question 1: See picture…
Question 2: could you please give me some advices to do this kind of game. I hope it looks realistic. I must use Physics, because I will have these things in my game:
32814-platform_rotate.png

I don’t know if i have to put it on answers or not.
but I suggest you to do it throw your 3d application
in 3d graphics each face has its normal which is usually perpendicular to it.
now imagine a plane which has some normals on it.
these normal couldn’t cover both sides
so if you add a mesh collider to it(don’t use convex) the side normals are facing at has collision and the other side doesn’t have.
in 3ds max for example you can easily inverse normals direction by selecting the face you want to change and from modifier panel select “Normal” (make sure the inverse normals is checked).

this answer is not engine side and you don’t need to write scripts or do fancy stuffs.

Maybe this could help you:

I try also to implement this kind of thing but i’m stuck in other thing for now…