Three spools `A, B` and `C` each having moment of inertia `I=MR^2/4` are placed on rough ground and equal force `F` is applied at positions as shown in the figures (a), (b) and (c). Then

Options

A. frictional force on spool `A` acts in forward direction
B. frictional force on spool `A` acts in backward direction
C. frictional force on spool `B` acts in forward direction
D. frictional force on spool `B` and `C` acts in backward direction

Answer

A, D

Explanation

For figure `A` and `B`
If `k^(2)gtRx` friction will act in backward direction otherwise in forward direction.
For given spool `K^(2)=(R^(2))/4`
Figure a `Rx=R.R/3=(R^(2))/3` friction forward
Fig b `R=R.R/4=(R^(2))/5` friction backward.
In figure c as force is below the diametric plane, hence friction backward.