8.3. Two Conditions of Equilibrium http://www.ck12.org
8.3 Two Conditions of Equilibrium
Objective
The student will:
- Understand the necessity for two conditions of equilibrium to ensure static equilibrium.
Vocabulary
- rotational equilibrium:A state in which net torque is equal to zero.
- static equilibrium:The state in which a system is stable and at rest. To achieve complete static equilibrium,
a system must have both rotational equilibrium (have a net torque of zero) and translational equilibrium (have
a net force of zero). - translational equilibrium:A state in which net force is equal to zero.
Introduction
The word equilibrium means balance. In particular, static equilibriummeans that a system is stable and at
rest. This means that the net force must be zero –calledtranslational equilibrium. However, to be complete the
net torque must also be zero –calledrotational equilibrium. For complete static equilibrium, both of these two
conditions must be fulfilled.
Consider the Free Body Diagram (FBD) in theFigure8.7. A meter stick is lying on a smooth flat surface with two
equal and opposite forces acting upon it. It is clear that the horizontal forces∑Fx=Fnet x=0 and the vertical forces
∑Fy=Fnet y=0, and therefore, the meter stick is either at rest or moving with constant velocity. We will assume
that it is at rest.
FIGURE 8.7
We will now apply the forces to the ends of the meter stick.