Blend generated fbx import to unity problem

I have create fbx file (It include simple cube) by blend. It import well and I add that prefab to roll a ball project unity 5.1.

When I play it I have noticed that the ball doesn’t hit the cude. Ball go through the object. What the reason for that? How to solve this?

Im new to Blend and Unity3D too

add a rigidbody to both the cube and the sphere.
Make sure the cube has a box collider and the sphere does too.

If there is a floor make sure it has a collider as well