Direction of friction at various positions in a circular track A small marble is rolling without slipping in a circular track (pic enclosed) in a vertical plane. Marble has enough energy to reach the top and not fall and complete the full track.  I am trying to understand the motion and hence trying to find direction of friction at various points on it. I have depicted what I think is the direction. Few questions:

*

*What would be the direction of friction at point A and C.

*At point B and D which force provides for Normal ? is it friction ?

*Between A & B and B & C and C & D and D & A, is component of mg providing for Normal?


 A: Imagine for a moment a marble is rolling along a track and it hits a patch of ice (no friction).  What changes?  For the most part, nothing.  It continues moving forward, and it continues spinning.  An observer would say it is still rolling, but there's no forces that bind the two together any longer.
If the marble were to go up a hill, the normal force would slow the forward motion, but would not (as long as friction were still absent) slow the spinning motion.  It would be spinning "too fast".   If it then reached a point where the surface had friction, forces would appear that would simultaneously slow down the spinning motion and speed up the motion.  Friction appears in a way that opposes what would otherwise be the marble's acceleration direction.  At spots where there is no (tangential) acceleration, there is no friction in the ideal case.
Normal forces arise to oppose any motion that would take the marble into the surface of the track.  It is necessarily not friction as friction always acts tangentially to the surface, while normal forces are perpendicular.
A: Although you do not state this, I am going to assume from the direction of the weight vectors on your diagram that the plane of the circular track is vertical, not horizontal.
At any point on the track there are three forces acting on the marble: its weight, the normal force from the track and friction.
These are three separate forces. The marble's weight $mg$ always acts vertically downwards. The normal force $N$ always acts perpendicular to the track - since the track is a circle, it always acts towards the centre of the circle. Friction always acts in the opposite direction to the marble's velocity. Since the marble is travelling anticlockwise around the circle, friction always acts along a tangent to the circle in the clockwise direction.
The normal force is not "provided for" by any other forces - it is simply the force exerted on the marble by the track that stops the marble from falling through the track. The magnitude of the normal force will depend on how fast the marble is travelling around the track, and on where the marble is on the track.
At $C$ the normal force minus the weight of the marble has to equal the centripetal force required to keep the marble moving in a circle. At $A$ the normal force plus the weight has to equal the centripetal force. Note that the normal force from the track cannot become negative (although it could if the marble was a bead sliding on a wire), so if the weight of the marble exceeds the centripetal force required then the marble will fall off the track somewhere between $D$ and $A$.
