I didn’t mention this clearly, but the FBDs drawn were for before the pendulum would start swinging (aka when the pendulum is at rest in its respective frame of reference). The goal was to demonstrate that the apparent gravitational field strength inside the elevator is g + a, which applies when the pendulum is swinging as well. I am not an expert at all with fictitious forces/non-inertial reference frames so please correct me if I messed anything up
I didn’t mention this clearly, but the FBDs drawn were for before the pendulum would start swinging (aka when the pendulum is at rest in its respective frame of reference). The goal was to demonstrate that the apparent gravitational field strength inside the elevator is g + a, which applies when the pendulum is swinging as well.
I am not an expert at all with fictitious forces/non-inertial reference frames so please correct me if I messed anything up