why sphere stop?

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I made rail in blender. And I gave 0 Friction to all of them. first time sphere go up and down. An then it stops slowly. I don’t have any idea of this situation. I want that sphere doesn’t stop.

Your sphere is affected by gravity. If you want it run in circle you can either use Transform.RotateAround, or set rigidbody’s drag to 0 and Use Gravity to false and then at Start() add force using Rigidbody.AddForce

float speed = 9.81f;
void Start()
    GetComponent<Rigidbody>().AddForce(Vector3.down * speed);