
Restoring force $F = KA = 2ma$ $⇒$ $a = \frac{{KA}}{{2m}}$
There will be no relative motion between $P$ and $Q$ if pseudo force on block $P$ is less than or just equal to limiting friction between $P$ and $Q$.
i.e. $m\;\left( {\frac{{KA}}{{2m}}} \right) = $ Limiting friction
$\therefore $ Maximum friction $ = \frac{{KA}}{2}$



