Hi,
i add a force with AddForce to my rigidbody and the rigidbody will move fine. But if the rigidbody touches another object and i will add a force the force will be lower as if the rigidbody wouldnt touch the object.
How can i add a independently force to a rigidbody which is not constrained on the surface the rigidbody is touching ?
bump
Not really sure what you mean here. Are we talking about friction here or is the rigidbody you are adding force to pushing another object?
yes friction 
How to avoid it ?
I dont know how to avoid it, but you can make a physic material and set dynamic and static friction very low ( or 0) and add that to the surface in which you collide with.