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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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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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