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Chapter 12
Static Equilibrium
and
Elasticity
Static Equilibrium
•Equilibrium implies the object is at rest (static)
or its center of mass moves with a constant
velocity (dynamic)
•Static equilibrium is a common situation in
engineering
•Principles involved are of particular interest to
civil engineers, architects, and mechanical
engineers
Equilibrium Summary
•There are two necessary conditions for equilibrium
•The resultant external force must equal zero:
ΣF = 0
•This is a statement of translational equilibrium
•The acceleration of the center of mass of the object must be
zero when viewed from an inertial frame of reference
•The resultant external torque about any axis must
be zero:
Στ = 0
•This is a statement of rotational equilibrium
•The angular acceleration must equal zero
Static vs. Dynamic Equilibrium
•In this chapter, we will concentrate on static
equilibrium
•The object will not be moving
•Dynamic equilibrium is also possible
•The object would be rotating with a constant
angular velocity
•In either case, the Στ = 0
