The tube AC from a quarter circle in a vertical plane. The ball B has an area of cross- section slightly smaller than that of the tube, and can moves without friction through it. B is placed at A and displaced slightly. It will

The ball has just potential energy at point A. The gravitational force on the ball is the only force acting on it at point A. The tube's inner wall will be the target of this force, which is downward. The ball thus comes into contact with the inner wall of the tube at point A.

At point C, the potential energy has changed into kinetic energy after exiting the tube. The ball has some kinetic energy, which indicates some velocity. 

We know that a body in circular motion always encounters a centrifugal force that seeks to push it off of its circular route. 

This is the case with the ball because it is going in a circle. As a result, the ball will press against the tube's exterior wall, coming into touch with it.

The ball thus makes initial touch with the inside wall before eventually making contact with the outer wall.