The load $L$ is being elevated by the downward velocities of ends $A$ and $B$ of the cable. Determine the magnitude of the acceleration of point $P$ on the top of the sheave for the instant when $v_{A}=2 \mathrm{ft} / \mathrm{sec}$ $\dot{v}_{A}=0.5 \mathrm{ft} / \mathrm{sec}^{2}, v_{B}=3 \mathrm{ft} / \mathrm{sec},$ and $\dot{v}_{B}=-0.5 \mathrm{ft} / \mathrm{sec}^{2}$