Answer
a. The weight of the wall and the small amount of dirt underground to the left of the wall provide clockwise torque.
b. The additional weight of the dirt on the underground edge of the second wall provides greater clockwise torque.
Work Step by Step
a. Assume the wall would pivot around the lower left corner of the wall shown in the picture. The weight of the wall, acting at the CG provides a clockwise torque. The small amount of dirt pushing at the base on the left also provides a small clockwise torque. These torques help to keep the wall upright against the counterclockwise torque provided by the ground pushing on the right.
b. Adding the extension provides 2 factors to help keep the wall upright. The CG of the second wall is far to the right of the pivot point, so the clockwise torque caused by gravity will be quite large. Also, the weight of the dirt on the new extension of the wall provides a large clockwise torque, stabilizing the wall.
