a. If the object is not moving relative to the surface it's in contact with. the friction force is static friction. Draw a free-body diagram of the object. The direction of the friction force is such as to oppose sliding of the object relative to the surface.

b. If the object is slicing relative to the surface, then kinetic friction is acting. From Newton's second law, find the normal force n. The friction force is then directed opposite to the motion, and its magnitude is fk= Âµkn.
c. If the object is rolling along the surface, then rolling friction is acting. From Newton's second law. find the normal force n. The friction force is then directed opposite to the motion. and its magnitude is fr= Âµrn.

a. The free body diagram for this object has been attached. It shows all the forces acting on the body at rest, including the friction force in the opposite direction to sliding of the object (assume it's left to right).

b. Since the object is in contact with the surface, there is a normal force acting on both of them and is equal to the weight exerted by each. This perpendicular force is defined by Newton's second law of motion.

c. The force of friction always acts in a direction opposite to the direction of motion of the body. F = mg ('a' for acceleration is replaced by 'g' gravity because acceleration in this case is just gravity).

Hope that answers the question, have a great day!