The direction of the frictional forces is something you need to think about carefully. The best rule of thumb is the following: the direction of the frictional force is such that it tries to keep the two surfaces from slipping. In each of these situations, what would be the direction of the frictional force? In cases of the inclines, consider left to be down and to the left (parallel to the incline) and right to be up and to the right (parallel to the incline).

1.A coffee mug is resting on the top of a car. The car is moving to the right and accelerating to the right. Consider the frictional force of the roof of the car on the coffee mug. Left, down, right, or up?

2.A book is being pressed against the wall as shown to keep it from falling. The book is stationary. Left, down, right, or up?

3. Consider the frictional force between the book and the wall. A block is placed on an incline as shown and the block doesn’t move.Left, down, right, or up?

4. Consider the frictional force between the block and the incline. Presume the surface of the incline to be more horizontal that vertical.

A block is being pushed to the right along an incline as shown. Left, down, right, or up?

5. Consider the frictional force between the block and the incline.
A coffee mug is sliding to the right across a tabletop. Left, down, right, or up?