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A disc is pulled by a force F acting at a point above the centre of mass of the disc. The direction of frictional force f acting on the disc pushed on a rough surface will be represented by

Which one is the correct ?

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Assume no friction .
Then for translating motion
$F= Ma_{cm}$-------(1)
Writing torque equation about COM
$Fr= I \alpha$
$F= \large\frac {Mr^2 \alpha}{2 r}$
$\quad= M \bigg( \large\frac{r \alpha}{2}\bigg)$--------(2)
Comparing (1) and (2)
$a_{cm}=\large\frac{r \alpha}{2}$.
Since $a_{cm} < r \alpha,$ there will be backward slipping and friction will act in the forward direction
answered Dec 5, 2013 by meena.p
edited Jun 22, 2014 by lmohan717

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