Gunman said:
Hm.. nrqed. I understand why you say that frictional force is same direction. Bt why then convectionally in a free body diagram the frictional force acting on a body is drawn opposite to the direction of motion?
Not always, no. It depends a lot on the situation!
For example, consider the following case which involves motion along a straight line instead of circular motion. You have a block of mass m on top of a block of mass M. Someone pushes on the lower block (M) in such a way that the two blocks are accelerating without the top block sliding. If you draw the free body diagram of the top block (of mass m), you will have a situation where the static friction force is in the same direction as the motion of the block.
The upshot is that it is simply not always true that friction is opposite to the motion. And as the example I just mentioned showed, you may even have motion and yet have a *static* friction force involved!
But one thing *is* true. A kinetic friction force will indeed always be opposite to the motion relative to the surface on which the object is moving. *That* is true but it can actually be subtle. I won't go into that for now.
As for static friction force, the rule of thumb to find the direction is to ask the following: imagine that friction would be completely removed, in what direction would the object slip (it's not always obvious what the answer is but let's assume that the answer is obvious). Then the static friction force will be opposite to that. In the case of the car in UCM )on a flat surface, not a banked one), then it is clear that if the road is completely icy, the car will move away from the center. So the static friction force in toward the center. But again, another argument is that there must be a net force toward the center in UCM and the only force available for that here is the static friction force.
Hope this helps a bit.