The railcar docklight is supported by the $\frac{1}{8}$ -in.-diameter pin at $A$. If the lamp weighs 4 lb, and the extension arm $A B$ has a weight of $0.5 \mathrm{lb} / \mathrm{ft}$, determine the average shear stress in the pin needed to support the lamp. Hint: The shear force in the pin is caused by the couple moment required for equilibrium at $A$