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In a question, its given that a plank is resting on 2 rollers like:

enter image description here

Then the explanation, as part of computing the forces, for the direction of friction on the rollers look like:

enter image description here

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)

enter image description here

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
up vote 1 down vote accepted

RedGrittyBrick's comment is correct. In order to make a roller turn, there has to be force whose vector does not go through the roller's axis. It needs to avoid the axis in order to exert a turning moment. The only such forces are the frictional ones from the plank and the ground.

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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.

(As a side note, associating direction with static vs kinetic friction is tricky - I'm pretty sure I can construct examples of both.)

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