The direction of the frictional forces is something you need to think about care
ID: 1408024 • Letter: T
Question
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).
A coffee mug is sliding to the left across a tabletop
A book is being pressed against the wall as shown to keep it from falling. The book is stationary.
Explanation / Answer
The direction of the frictional force is always opposes the motion of the object or surface.
If a block slides along the plane, the friction force is opposite to the motion of the block. This mean along the suface of the ground not the surface of the block.
In case of the inclined plane, the friction force is along the inclined in the upward direction (parallel to the incline).