Question

A ground retaining wall is shown in Fig. 9–36a. The ground, particularly when wet, can exert a significant force F on the wall. (a) What force produces the torque to keep the wall upright? (b) Explain why the retaining wall in Fig. 9–36b would be much less likely to overturn than that in Fig. 9–36a.

Short answer

a) Gravitational force produces the torque to keep the wall upright.

(b) Due to the higher mass of the second wall, the torque produced by gravity (to hold the wall upright) will be greater for the second wall.

Step-by-step solution

Understanding torque

The term torque may be defined as the quantity that estimates the capacity of a force to twist an object about some fulcrum. It is also referred to as the moment of force.

Force that produces the torque

In the case of (a), if the ground exerts a force on the wall, the gravitational force acting through the center of mass of the wall will produce enough torque on the wall to keep it upright.

Explanation for why the retaining wall in Fig. 9–36b would be much less likely to overturn than that in Fig. 9–36a

(b)

In the case of (b), the wall's mass is large, and the torque through the center of mass will be sufficient enough to keep the wall upright.

The ground above the extended part of the wall will provide a clockwise torque on the wall. Moreover, the center of mass of the total wall is to the right of the rotation point of the wall, and the gravitational force acting through the center of mass of the wall exerts a clockwise torque.

Thus, in case (b), the wall is less likely to overturn.