In a question, its given that a plank is resting on 2 rollers like:
Then the explanation, as part of computing the forces, for the direction of friction on the rollers look like:
Why are the direction this way? Since the rollers are turning clockwise (plank is moving right), I expected the opposite?
Explanation on the directions look like: (see line 5 counting from bottom)
Although, after the computation, the direction of $f_b$ is pointing to the right as its negative.