* Friction opposes motion: Friction always acts in the opposite direction of the object's motion *or* the intended motion.
* Direction Matters: The sign of friction (positive or negative) depends on your chosen coordinate system.
* Free Body Diagrams: Free body diagrams illustrate all forces acting on an object. Friction is simply one of those forces, and its direction is crucial for understanding the object's behavior.
Example:
Imagine a box sliding to the right on a rough surface.
* Coordinate system: If you choose right as positive and left as negative, then friction will be acting to the left, making it a negative force.
* Coordinate system: If you choose left as positive and right as negative, then friction will be acting to the right, making it a positive force.
In conclusion: The sign of friction on a free body diagram depends entirely on your chosen coordinate system. Focus on the direction of friction relative to the object's motion, and then assign the appropriate sign based on your coordinate system.
