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# 12: Static Equilibrium and Elasticity

In earlier sections, you learned about forces and Newton’s laws for translational motion. You then studied torques and the rotational motion of a body about a fixed axis of rotation. You also learned that static equilibrium means no motion at all and that dynamic equilibrium means motion without acceleration. In this section, we combine the conditions for static translational equilibrium and static rotational equilibrium to describe situations typical for any kind of construction. What type of cable will support a suspension bridge? What type of foundation will support an office building? Will this prosthetic arm function correctly? These are examples of questions that contemporary engineers must be able to answer.

• 12.1: Prelude to Static Equilibrium and Elasticity
The elastic properties of materials are especially important in engineering applications, including bioengineering. For example, materials that can stretch or compress and then return to their original form or position make good shock absorbers. In this chapter, you will learn about some applications that combine equilibrium with elasticity to construct real structures that last.
• 12.2: Conditions for Static Equilibrium
A body is in equilibrium when it remains either in uniform motion (both translational and rotational) or at rest. Conditions for equilibrium require that the sum of all external forces acting on the body is zero, and the sum of all external torques from external forces is zero. The free-body diagram for a body is a useful tool that allows us to count correctly all contributions from all external forces and torques acting on the body.
• 12.3: Examples of Static Equilibrium
In applications of equilibrium conditions for rigid bodies, identify all forces that act on a rigid body and note their lever arms in rotation about a chosen rotation axis. Net external forces and torques can be clearly identified from a correctly constructed free-body diagram. In setting up equilibrium conditions, we are free to adopt any inertial frame of reference and any position of the pivot point. We reach the same answer no matter what choices we make.
• 12.4: Stress, Strain, and Elastic Modulus (Part 1)
External forces on an object cause its deformation, which is a change in its size and shape. The strength of the forces that cause deformation is expressed by stress. The extent of deformation under stress is expressed by strain, which is dimensionless. Tensile (or compressive) stress, which causes elongation (or shortening) of the object or medium and is due to external forces acting along only one direction perpendicular to the cross-section.
• 12.5: Stress, Strain, and Elastic Modulus (Part 2)
Bulk stress causes a change in the volume of an object or medium and is caused by forces acting on the body from all directions, perpendicular to its surface. Compressibility of an object or medium is the reciprocal of its bulk modulus, the elastic modulus in this case. Shear strain is the deformation of an object or medium under shear stress. Shear stress is caused by forces acting along the object’s two parallel surfaces.
• 12.6: Elasticity and Plasticity
An object or material is elastic if it comes back to its original shape and size when the stress vanishes. In elastic deformations with stress values lower than the proportionality limit, stress is proportional to strain. An object or material has plastic behavior when stress is larger than the elastic limit. In the plastic region, the object does not come back to its original size or shape when stress vanishes but acquires a permanent deformation. Plastic behavior ends at the breaking point.
• 12.E: Static Equilibrium and Elasticity (Exercises)
• 12.S: Static Equilibrium and Elasticity (Summary)

Thumbnail: Balanced Rock in Garden of the Gods. (CC BY-SA 2.5; Ahodges7).