Two particles P and Q are moving on a circle. At a certain instant of time both the particles are diametrically opposite and P has tangential acceleration $8 \, m/s^2$ and centripetal acceleration $5 \, m/s^2$ whereas Q has only centripetal acceleration of $1 \, m/s^2$. At that instant acceleration (in $m/s^2)$ of P with respect to Q is :