# Direction of friction on rollers with plank resting on top

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.

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There's no other horizontal force on the rollers, so the frictional force from the rightward moving plank is what is making the rollers move to the right. The opposing frictional force from the ground therefore causes rotation (otherwise the rollers would almost glide to the right) The diagram is therefore correct. – RedGrittyBrick Dec 3 '12 at 15:26

When you say you expected the friction to be moving in the opposite direction as the plank, I assume that's because you expected the friction to be kinetic. In fact, as the previous posters have mentioned, there is no $relative$ motion between the plank and the rollars - rolling without slipping - and so we have static friction, not kinetic.