Rigidbody constant speed + gravity.

Questions & Answers
1

I have an object which is affected by gravity and collision effects. How can I make it to maintain a constant velocity on the X axis?
I have something like that:

void Update () {        
     rigidbody.velocity = 5 * new Vector3(1f,0f,0f);
}
2

gameobject.velocity = vector3(constantX,gameobject.velocity.y,gameobject.velocity.z);