Answer
(a) and (c) are in equilibrium
Work Step by Step
İn order to find which disks are in equilibrium, we need to write 2 conditions:
(1) net force must be zero
(2) net torque must be zero
Therefore, we write force and torgue equations relative to center for every condition:
$a)\sum \overrightarrow {F}=3F-2F-F=0.
$
$\Sigma \overrightarrow {t}=F\times R-2F\times \dfrac {R}{2}=0$
So disk is in equilibrium
$b)\Sigma \overrightarrow {F}=2F+F+F=4F\neq 0$
Disk not in equilibrium
$c)\Sigma \overrightarrow {F}=F+F-2F=0$
$\sum \overrightarrow {t}=F\times R-F\times R=0$
So disk is in equilibrium
$d)\sum \overrightarrow {F}=2F-F-F=0$
$\sum \overrightarrow {t}=2F\times R-F\times R=F\times R\neq 0$
So disk is not in equilibrium
