how would you make a object fall though a circle(check the link)
ive tried tutorials on youtube, but they cover mostly for the unreal engien
this, how do you make a custom collision for this. i have tried meshcollisionshape and everything, cant get it to work 
Have you tried
createDynamicMeshShape(Spatial spatial)
from: http://hub.jmonkeyengine.org/javadoc/com/jme3/bullet/util/CollisionShapeFactory.html
If not, as it says there, you can try
GImpactCollisionShape(Mesh mesh)
from: http://hub.jmonkeyengine.org/javadoc/com/jme3/bullet/collision/shapes/GImpactCollisionShape.html
But for what its worth, with things like that its often better to just create your own collision shapes in your modelling software
@Ben255 said: this, how do you make a custom collision for this. i have tried meshcollisionshape and everything, cant get it to work :/
A mesh collision shape should work fine
yes ive used createDynamicMeshShape(Spatial spatial). but the walls are not curved. it still becomes a square and ends up in the middle of the road. ive tried to find a tutorial to make own collision shapes in blender but cant figure it out. UCX_name and that, but that seems to be for the unreal engien.