1. Consider the forces acting on the object:
* Gravity: Always acts downwards.
* Normal Force: Perpendicular to the surface, pushing the object upwards.
* Friction: Opposes the motion or tendency of motion.
2. Determine the direction of motion or impending motion:
* If the object is sliding down the incline: The friction force points up the incline.
* If the object is being pulled up the incline: The friction force points down the incline.
* If the object is at rest: The friction force will point up the incline if the object is *about to slide down* and down the incline if the object is *about to slide up*.
Key point: Friction always acts to oppose relative motion between the object and the surface.
Example:
Imagine a block sitting on an inclined plane.
* If the block is sliding down: Friction points up the incline, trying to stop the block from sliding down.
* If the block is being pulled up: Friction points down the incline, trying to prevent the block from moving up.
* If the block is at rest: If the block is about to slide down, friction will point up the incline. If the block is about to slide up, friction will point down the incline.
