Homework H$\mathrm{.}5.$E

Given: A thin homogeneous bar OA $($of negligible mass and of length $L)$ is pinned to ground at O $.$ The other end of the bar is pinned to a second thin homogeneous bar AB $($of mass $m$ and length $2L)$ at A $.$ End B of bar AB is able to slide on a smooth, horizontal surface. At the instant of release from rest, OA is horizontal and end B of bar AB is located directly below O $.$

Find: Determine the angular acceleration of bar AB on release. Write your answer as a vector.

Use the following parameters in your analysis: $m=50kg$ and $L=3m.$