Definition:Equilibrium (Mechanics)

This page is about equilibrium in the context of mechanics. For other uses, see equilibrium.

Definition

Let $B$ be a particle, a system of particles, or a body.

Let $B$ be such that:

the resultant of the external forces acting on $S$ is zero

the sum of all the moments of all the external forces acting on $S$ is also zero.

Then $B$ is said to be in equilibrium.

Let $B$ be a particle, a system of particles, or a body which is in equilibrium.

Let $B$ be such that if a sufficiently small displacement is applied, then it returns to its original position.

$B$ is then said to be in stable equilibrium.

Let $B$ be a particle, a system of particles, or a body which is in equilibrium.

Let $B$ be such that if a displacement is applied, however small, $B$ moves to a position different from its original position.

$B$ is then said to be in unstable equilibrium.

Let $B$ be a particle, a system of particles, or a body which is in equilibrium.

Let $B$ be such that if a small displacement is applied, $B$ remains in its new position.

$B$ is then said to be in neutral equilibrium.

Results about equilibrium in the context of mechanics can be found here.